• Standard plenum but been converted to forward facing - issue, likely to have flow problems
• Ford XF 80mm throttle body – not required, stock is fine upto 400rwkw. Going larger just creats throttle control issues
• 3 inch turbo back high flow exhaust system - issue - you need at least 3.5"
As for the rest, 290rwkw @ 20psi from a GT35, you have a serious problem.
A GT35 will make 360-370rwkw comfortably on 20-24psi usually.
Exhaust is probably a cause.
You make no mention of what exhaust manifold either, or external gate?
It looks like a tiny rear housing from the small pics which would suggest a .63 rear, which is a bad choice for such a turbo.
So change that, along with exhaust. Decent manifold and gate and you'll be set.
That'll come at a cost of around 5k though, so its not exactly a cheap return