The good
After much debate on pistons, I decided on a set of Mahle 99.5mm forged. I liked the fact that you can run tighter clearances. They say .0020" is ok, but I went with .0026" with the bores individually honed to match to their respective pistons.

To my surprise, they sound like stock! The first ~15 seconds or so I hear a very SLIGHT slap (more like normal noise, but a tad more pronounced) but then it goes away. I couldn't hear it from in the car, but only when I had my head under the hood. Definately not any diesel sound.

The bad
I do have a complaint about the Mahle kit though. The main problem is the wrist pin retaining clips. Unlike the stock ones, they do not have tangs, so installing them is an extreme PITA. You have to work one end into the piston then walk the rest in. You can easily spend 20-30 mins working on a single clip (1 out of 8!). I found the best way to make this as quick as possible is to remove the #2 and #4 rods from the crank so you can put the clips on both sides of the piston while they are out of the block. Then push the rods through the block and bolt to the crank (just how you do normal, non-boxer motors). If you do it this way, then you only have 2 clips to install while the shortblock in assembled (#1 and #3).

I have to give BIG thanks to Dan and Jeff at Godspeed/Tuning Factory. They went beyond the sale and provided very good technical support.

The ugly
Some pics:

Stock vs Mahle:
http://www.cottrell.100megs6.com/CJB/tech/EJ257/Mahle/pistons3.jpg

http://www.cottrell.100megs6.com/CJB/tech/EJ257/Mahle/crank_and_piston.jpg
Almost there:
http://www.cottrell.100megs6.com/CJB/tech/EJ257/Mahle/shorty1.jpg

All done:
http://www.cottrell.100megs6.com/CJB/new_age4/front_fm_angle.jpg

The motor is done now. I decided to use the stock 257 rods as they more than meet my power goals. The only thing I didn't do that I would like to is to go to shimless buckets so I can use my launch control without having to worry about spitting a shim.

I had my injectors cleaned and flowed. They are a true 840cc now. I also added a Crucial Racing T-stat, which hopefully make a big difference as far as safety and det (I had a head crack in the past from heat). The cooling fans rarely even come on now with the T-stat. :D

Next step, GT30R (52lb), 6spd, and a beater.

More to come....
[Oh, setup is WRX heads milled .006", EJ257 shorty decked .003", 8.78:1 CR, UTEC, Tuna, TBE, FMIC, UP, modded injectors, fueling, plus normal bolt-ons/suspension, etc]

Very nice!!!

The good
After much debate on pistons, I decided on a set of Mahle 99.5mm forged. I liked the fact that you can run tighter clearances. They say .0020" is ok, but I went with .0026" with the bores individually matched to their respective pistons.

Wow, you sure about that clearance? Clearance is largely based on the type of aluminum used in the forging. .0020" sounds kinda scary to me. Some of the best forgings I am aware of can't run below .0030" clearance.

BTW, piston companies also specify different measuring points for clearance. I know that Wiseco for instance specifies measuring near the bottom of the skirt (apparently most of the expansion happens there (?) ). Other specify at the wrist pin. I hope you had the right measuring point. :p

Wow, you sure about that clearance? Clearance is largely based on the type of aluminum used in the forging. .0020" sounds kinda scary to me. Some of the best forgings I am aware of can't run below .0030" clearance.

BTW, piston companies also specify different measuring points for clearance. I know that Wiseco for instance specifies measuring near the bottom of the skirt (apparently most of the expansion happens there (?) ). Other specify at the wrist pin. I hope you had the right measuring point. :p
Yes, the Mahle pistons are a bit harder than the CP's and are coated. Several people are running .0025" here. The Mahle's are tapered and the skirt is the widest point. I'm .0006" over the min spec. Thanks for the thoughts.

Mahle recommends 0.0025, but does not specify a boost/HP level. There's always a game played with manfacturer's tolerences--how conservative are they? Regardless todays CNC-crafted forged pistons are not your father's forged pistons.

Mahle has a Grafel (lubricating, slick) coating on the skirts and the entire pistion has been phosphated.

pardon my ignorance, but what happened to the old block?

I understand you were running a 2.5 setup, and it looks like you lost a cylinder... So did you get a new 2.5 shortblock and just drop the pistons in? (I was under the impression that this is not a good idea to just drop pistons in...) Can you please enlighten me here?

again, please pardon my ignorance... I might be doing something similar in future and am trying to take in all i can here...

and just drop the pistons in? (I was under the impression that this is not a good idea to just drop pistons in...) Can you please enlighten me here?
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I went with .0026" with the bores individually matched to their respective pistons.
I had the bores sized to fit the pistons.

I finished the break in and did some prelim runs. To my surprize, I didn't need to relax the knock parameters at all.

I did a compression test and I was 170-170-170-185, with #4 being the high one. That is odd.

With the stock STi pistons, it was 180-180-180-185.

The bad part is that I didn't prime the turbo and it SCREAMS. :(

Nice kill on the thermostat....many people overlook that very important piece. :) Best of luck.

Sounds like you have a great setup going for you. Glad to see that things are looknig up after your other scammer motor troubles.

What gasket / thickness did you use to achieve your desired CR....I ask because the Mahle's are listed as 8.5:1 compared to the stck piston's 8.2:1......and with the WRX heads added - ????

The bad part is that I didn't prime the turbo and it SCREAMS. :(

was this a new turbo, or the same old one?

What gasket / thickness did you use to achieve your desired CR....I ask because the Mahle's are listed as 8.5:1 compared to the stck piston's 8.2:1......and with the WRX heads added - ????
Stock STi gaskets, block decked .003", heads milled .006".

was this a new turbo, or the same old one?
I had a tear in my turbo inlet hose, so I swapped that out and replaced my uppipe with a gutted stocker (I've swap back and forth sometimes).

The combination of the two made the sound go away. The turbo is fine, zero whine or shaft play.