My 996TT ( Rs-tuning stage 2 542 PS ) runs great when full tank of gas , but it suffers in loss of power when fuel level drops in the Tank.
This power loss starts at about 2/3 of Tank and becomes more significatively at half tank or below .
Unfortunately i Have no instruments to datalog fuel pressure , AFR and other parameters , but for example my 100 - 330 Km/h time ( same road and weather conditions ) is about 2 seconds slowly when running half tank instead a full tank of gas .
I'm pretty sure the lower fuel level in tank cause a fuel pressure drop and the car runs leaner with consequently loss of power especially at top end .
But i don't kmow why , weak fuel pump , leakage in the line ... ?
Anyone with this problem . Maybe a new fuwl pump can solve the problem ?

 

inner fuel tank flex hose split?

 

That was my guess also- but just a guess. Needs to check fuel pressure.

 

Pull the fuel pump module and examine all the hoses for cracks. It's really not too hard to get the module out and if you find no issues pull the octopus hoses and you'll likely find the problem. From your description the issue has to be in the tank...