The FT isnt big enough for 300hp IMO.
I think that 300hp might be possible, but you'd need to convert to using an S3 compressor housing, and use a 2280 or 2283 ETT wheel, like Bills K03+ hybrid.
As you've pointed out, you still need all the right bolt ons, You need a decent exhaust, preferably 3" from the turbo back. You need a FMIC, injectors, maf etc.
The cost of the turbo is the only real difference between a GT28RS when you draw it all out, but i think the more important thing to consider is that with the hybrid approach, you can do it all in stages over time, then fit the turbo at the end once everything else is in place.
Whereas going GT28RS means you basically need to do everything at once.