Maybe just head over to SPAL's site and see what they have that is an improvement over stock that fits in the same shroud. From what I hear though, the fans that came with the Z are pretty nice. If you already have a CSF radiator and you have an intercooler up front, maybe create some ducting on a Z speed undershroud that wind.zero did:
DIY Zspeed Nismo370Z aluminum under pan diffuser but point the fresh air towards the radiator. Not sure how well it'd work but it was a thought. I know the 4th gen F bodies ran something similar because the limited cooling (if any) coming from the front bumper.
Edit: looks like someone beat me to suggesting SPAL fans for oem shroud lol.