With reduced aero, zero toe angle, a high, soft suspension and a VERY good draft, I was only able to hit ~230mph in the Sarthe1 race of Formula GT. It's not designed to have an uberhigh top speed, but to get to ~200mph VERY quickly. It's also meant to handle very well at speed, thanks to a precisely-engineered ultralight chassis and powerful downforce elements.
If you're wanting to take the FGT to the Nürburgring, you should start with a suspension that at maximum height, with a very soft spring ratio. Shocks should be an afterthought, as well as stabilizers. Maximum aero would help keep you planted at speed, as well as minimizing your jumps when you crest certain hills. Sub-5min laps have already been done, so it IS possible.