I had the same kind of problem with my stock 2.0L engine about 18 months ago. I decided to do the ej257 swap as well, keeping the stock heads, and the stock turbo. After a bit of searching around, I found the general opinion was my car would pull real hard down low, but fall flat on its face at about 5300 rpms. This is because the 2.5L block flows more air then the stock heads can handle. There were three solutions to fix this. First i could do the AVACS heads as well. This was to pricey for me. Second was head work on my stock heads, which was a bit expensive also. The third option was to get cams that would increase the lift of the valves to let more air in and out of the heads. STI cams weren't too expensive or aggressive, and i could get them at my local dealership. It worked out well enough. You just have to have your mechanic tap and plug the holes in the cams used for AVACS.
I dyno at 226whp/248wtq and my car pulls hard all the way to redline.
Hope this helps.