Have you checked out the (late model) GS forum to see what those guys are putting down with a few mods?
I was suprised to see some getting 260+ at the wheels, that's pretty decent compared to the few early SC dyno runs that I have seen.

 

SCv8 - while the 2nd gen GS also has the 1UZ FE motor like the SC, starting in 1998 it is a 300HP rated version with VVT-I. Not at all the same starting point as the early SCs. (edit - you probably already knew this, but thought I'd post for the benefit of others who may be new to Lexus)

Bone stock they dyno usually between 225 - 245 rwhp. Exhaust mods (other than headers) do basically nothing.. 1 - 4 rwhp at most unless combined with an intake / ECU or intake / SAFC-II combo to drive air/fuel mapping.