If you don't like the GForce trans, how about an Astro Performance gearset added to your stock unit. They're proven, rated to 525hp-650hp and at just under $1,400, still about $800 less money than buying a new TKO...and again, you don't need to replace the shifter, bell housing, drive shaft, clutch mechanism, crossmember, mount, etc.
Those items alone are going to eat up $600. That makes for a pretty expensive swap. Not to mention the GForce and Astro Performance units have the option of a very highway friendly .59:1 O/D ration, that I don't believe is available for the TKO's?
Not bad mouthing the Tremec, but there are more cost effective alternatives.