I don't use the OTT for setting SS, you can do that through in-game settings and I've found that using the OTT mucks things up a bit, although I've no idea why.
Try setting Textures to high and everything else on Med including MSAA.
Particles and their density can be set to low, same for Grass & Track Detail as these are quite resource intensive, so setting to low will help considerable without taking away the effects.
Screen Dirt only makes sense if you are using the Helmet view I have found, something I recommend when driving Formula or modern cars where available.
There is a setting to turn Rain Drops on/off.
1.7 SS is pushing it a bit in my opinion, whatever your GPU, happy medium I found to be 1.4
It's all about trial and error really, try doing a GT3 10 lap race at Spar with 25 cars in heavy rain, it will push any machine to the limits but what an experience !
Might help, might not !