http://www.turbobygarrett.com/turbobygarrett/tech_center/tech_center.html The garrett Faq was a really good read. It is amazing...according to thier calculations to get 400 bhpp out of a 2.0 you would have to run around 26 psi vs the same hp for a 2.5 liter is 16.8 psi. Consequently for 500 bhp (at the crank) a 2.0 liter would have to run roughly 33 psi vs 23 psi for the 2.5 liter block. And that would only put you at low to mid 400's at the wheels!
http://www.squirrelpf.com/turbocalc/index.php?target_peak_power=500&engine_disp=151&target_af=11.5&bfsc=0.50&max_ic_loss=.5&rpm_redline=8000&rpm_peak_power=6500&rpm_max_boost=4000&rpm_min_boost=3000&vol_1=89&vol_2=97&vol_3=100&vol_4=95&intemp_1=90&intemp_2=120&intemp_3=135&intemp_4=140&pre_company_id=1&pre_family_id=1&calc=yes&company=1&family=1&product=21 Now this is a truely nice tool. If you do some of the basic math from the other page and try to plot it, itis ok, but this thing does all the math AND plots it for you. All you have to know is what HP you are shooting for and it does most of the rest. You have to kind of guestimate on some of the numbers, but very cool stuff.