Right now I'm Stage 1 on a stock 04 wrx, and am dying to go to stage 2. Here's the question: if I get an uppipe, downpipe, intercooler y-pipe and flash the 91 octane stage 2 map, how much hp will I REALLY have? The 280 that Cobb says or something less or more? The rest of the exhaust is staying stock for now.
Oh yeah, I also have a Perrin hi-flo air filter.(If anybody says they don't make any difference-they're nuts. I felt the difference right away)

you're probably looking at around 35-40whp increase going from stock to stage 2. best bang for the buck ever i think.

How much HP depends on way to many factors to list - the biggest factor being the type of dyno used and it's calibration.

Cobb's numbers are generally accurate and not particularly inflated.

What I was really curious about is how much does an uppipe change the hp of a stage 2 flash. Also, isn't the general consenses that an intercooler y-pipe is good for around 5 hp? How does that figure in?

reliable numbers or not, going from stock to stage 2 is night and day.

What I was really curious about is how much does an uppipe change the hp of a stage 2 flash. Also, isn't the general consenses that an intercooler y-pipe is good for around 5 hp? How does that figure in?

Mods are never cumulative. Ie: If a y-pipe makes 5hp on a stock car, and if a CBE makes 5hp on a stock car...it does not mean that putting on both will net you 10hp. Not a great example, but you get the idea.

UP is a necessary (IMO) supporting mod to a stage 2 setup on pre-06 WRXs. It may not show a lot of peak hp gained but your spool-up will be faster and there's no chance of sending the cat into the turbo.

I was running cobb stage 2 for a year before putting in a new turbo and reflash. You should get a new uppipe ASAP. It doesn't matter how much gain you could(not will) receive with the uppipe, it's all about the insurance. These are the steps to my reasoning:
1 - Prevent the cat from being sucked into the turbo.
2 - Allow ~200-400 rpm reduced spool time
3 - Allows for more airflow for future mods that will actually need it

As for actual HP gains.... none. because stage 2 is programmed to have the stock uppipe. But the potential is there to create more with a custom tune. Hope this helps!

EDIT: one more thing about the air filter... An air filter is still an air filter. I put on a new air filter and WHOA... I felt the difference too; but that was just because I had a clean air filter. I now have a K&N filter. When I clean it out every 10,000 miles, I feel the difference (especially with Gas milage) I just think you felt a clean/new air filter. Can you clean the Perrin filter like the K&N? (I would assume so).

Happy Modding!