Here is some stock and popular turbo info that I found.
Turbo---CFM----Racing HP-----Daily HP inducer
HX35----574------420----------330------ 56
HX40----763------560----------440------60
TP38-----767
T66------800-------560----------440-----66
GTP38R--897
B1/S300--900-------630----------495-----62
H2E------868------700----------550------63
HX50----1000-----700-----------550-----63,67
HX55----1050------735----------578-----67
HT3B----1085------770----------605-----76
GT42----1302-----------------------------75
T76-----1350------945----------743------76
HT60----1400------980----------770-----76
B2/S400-1500------1050---------825-----75
GT47---------------------------------------80
HX60-----1550------1085-------853----
TV81----1600-----------------------------85
T88------1750-------1225--------963-----88
HT4B-----1800------1260---------990-----88
Big Brother-1900---1330--------1045-----87
T91-------2000------1400--------1100-----91
HC5A/HX82-2450----1715--------1348----95
T100------2600-----1820--------1430-----100
T105-----2900------2030--------1595-----105
From what I have read, is that using two turbos will effectively doubles the cfm. So if you take the info of the cfm for a two Hx35s, that equals roughly 1248cfm. That is more air than a Hx55. I have seen Hx55 turbos dyno at well over 600hp. Now with a Cat 3208 pump pushing enough fuel and the turbos pushing close to 40psi, I see no reason at all that 500hp isn't achievable. For spool up, a set of Hy35s would be more ideal with their 9cm turbine housings. Hx35s usually come with 12cm or 14cm housings and would spool slower but would provide more top end flow. Also iirc HY35 and Hx35 turbos have the same or similar compressors. It all just depends what you want in your setup and where you want your power to come on at. Like was said before, compounds will provide more boost overall. But if you don't desire an insane amount of boost, parallel twins will work out well for you. They will spool slightly better than a big single and will make similar power to one of equivalent cfm...