i think the argument is really .. a GTR is a GTR.. you can never change that. Like a Porsche is a Porsche, etc
I'd argue that nothing ever stops you putting a GTR engine in the GTS-T and that gives you the best value. If anything you're in a better position, as while the engine is out it can be fully be rebuilt for 600hp+ then you can swap it in once its finished. You'll get a few $k for your old RB25DET, etc. Then the only difference is really the exterior isn't it, and lack of semi-4WD. Then you have to get it all to the ground.. bit easier with a GTR, but not unachievable with RWD.
A lot of people make mistakes along the way going for big HP, thats generally where i guess the costs add up.