BMW did mate a 6speed with the E60 M5, but it was only for the North American market. Same deal this time around, North American markets only again.
The 6speed version of the E60 has a lowered redline to protect the gearbox, traction control also cannot be switched off to protect the clutch, in another words, the manual version was a neutered one.
Transmission ratings are limit by torque, not HP.
Regarding Porsche's manual transmissions, Porsche is not a high volume maker, their highest output, specifically torque output engines are putting down 700Nm, pretty much still within spec for the Getrag box, another thing is, 911 weights a lot less than big sedans like M5, even at the same torque output, there will still be less stress on the transmission on the Porsches.