Correct me if I'm wrong, but both tunnels on SSR7 have the exact same grade....
Regardless of which tunnel you are in, if you are heading towards the toll booth turn-around, you'll be going up hill. If you are heading away from the toll booth turn-around, you'll be going downhill.
The point is that the OP's issue isn't necessarily related to whether or not GT6 is borked.
It is more likely highlighting the fact that GT6 may have got it quite right in the respect that an S-type R may be able to beat an XF-R due to the power being greater than traction.