I own a US Spec GS300, and im currently considering either adding a supercharger, turbo kit or just importing a J-Spec Aristo V300 Vertex engine. I would appreciate opinions and thoughts on what the best route for me to take would be.
Thanks in advance for your help.

its a hard to say really,it depends on costs

the turbo conversion would possibly be the cheapest
fitting the 2jz-gte lump would be the best route,but you will need the loom aswell

It is very hard to get a V300 engine to run correctly in a GS300, you will need all the electronics to control the engine and then you will have the problem of making it talk to the rest of the cars electronics.
It can be done, but it's a lot of work and very expensive.

You'll be cheaper off getting an SRT turbo kit.

I agree with Ritchie, it's a Supra Mk.iv engine plus a LOT of differences e.g.gearbox, ECU, steering wheel etc.

It would be a nightmare.

I advise either the turbo kit for the GS300 or buy a V300.