Well I can handle GTA 4 with iCEnhancer just fine so I believe I'll be able to play GTA 5 on nice graphics.
Which Icenhancer are you using, and Patch 4 or 7?
I'm still waiting for the 3C version. I feel like 3.0 is the best of the bunch with quality and performance, but I just couldn't get past the ugly light orbs and cones that appear at night. Apparently it's a texture/model thing that can be fixed, and it's going to be included in the 3C version.