I can't see that a GT28rs on a MX5 engine would produce 400bhp. You would be 50bhp short. You would need one hell of alot of work to get to 400bhp on a 5 engine.
Please tell me if my Cossie prices below are wrong but I dont see that it would cost much more.

MX5 Diff - £350 + shalfs
Cossie - £300 = shalfts

MX5 box - £450
Cossie - £300 + Prop

MX5 management - £400. Selling my megasquirt
Cossie - £200 for L6 ecu?

MX5 engine - £550 for later engine or £2500+ for fully forged, + £1000 for turbo + £500 for absurdflow manifold.
Cossie - £1000 for std engine with the intention to run a chip to get me to 250ish bhp. Tuning will come later.

I know there are other factors to consider but not large cost items.

All advice welcome.