That 478 is probably a pump gas tune. It has a fuel system so it is 650-700whp capable with that turbo.
That being said, unless it has had all the suspension, brake system upgraded to TT spec, the 6spd swap, and the rear end swap, you are dealing with weaker/shittier parts. Has a thicker head gasket been put on?
IMO I would keep the 7. If you are going to trade for a Supra, go with a turbo model from the start, unless someone has done all the above mentioned modifications. Then I MIGHT consider it.
NA-T Supra 15-20k. LS1 FD 25-30k. The last time I was actually paying attention to the market for the cars.