OK so you have a bigger turbo with more lag and less power on less boost. The cars should be tuned to the same margin of safety given the same tuner was used. So for less out-of-pocket you get a bit less performance. We all have budgets so I can understand going this route. BUT...
The way the car is setup now is with the tial 0.63 A/R. It is an absolute BLAST on the street and autocross. Although, it is not the highest HP setup we've had on the car and this is something we will be going back to, the 0.82 A/R.
Here's a plot from the first setup. It had a 44mm WG (now on 38mm), which I suspect was a bit sticky upon opening due to the torque dip you see as it reaches target boost, and it had the 0.82 A/R turbine housing. I know I know, it's not the same dyno and not the same tuner, but butt dyno confirms similar torque, but much more power up top. In fact with the 0.82 A/R we kept pushing the rev limit to 7.500RPMs until power started to taper down. The plots give a resemblance of this as well. For Max power and a car that would see road coarse duty I would without a doubt go with the 0.82 A/R housing or maybe even a 1.06 A/R. Plus we have the GTX3576 with the 0.63 A/R that Garrett tells me will spool like the 3071 with the 0.82 A/R, but make MUCH more power. Well see, still lots more testing to do!
Another benefit to this kit is in its versatility. The ATP turbos we're seeing here in the comparisons have A/R 0.73 IIRC and that's it. So say you're running someone's rotated setup. You have to pull header, up-pipe, down pipe and replace all those gaskets, OR pull the turbo and deal with oil coolant lines plus still changing gaskets. With out kit it should take a lot less time and NO gaskets. I will be doing this in the next few weeks, but if it goes according to plan, it will be 3 V-bands and you can swap the housings. We'll see how smoothly this goes in the coming weeks, but for those running street/track tunes this can be a significant advantage to the kit. Maybe?
While I REALLY enjoy street driving the car with the 3071 I'm not convinced yet that it's going to hit the sweet spot we are looking for, for this kit, on pump anyway. E85 and its a different story, but that's a debate for another day. My point is, this is a work in progress. I really enjoy sharing the info as we go and the feedback from the community. In the end if the kit isn't worth the price of admission, it won't go into production and I'll be able to enjoy it's marginally better performance all by myself