If all you want is more power, You can make your GT as powerful as a GTs by just changing several parts.
Get an Intake
, a header, an exhaust, a crank pulley, an altenator pulley, a flywheel, an ignition system, and a ECU system.
You will have a GT that is faster than a GTs for much less than the trading in cost.
And then you can do more things to it... like porting the head
Besides, if you want to get forced induction, the GT is a better base because the compression ratio is lower.