Download a few replays from LFSW and focus on the 2nd sector of Aston GP. It's the part where people make or lose time.
Both the XRR and FZR can behave well (and be somewhat quick) on varying setups, so I'd say it's more about making the most of your current setup by learning to drive it fast than seeking the best setup.
FZR is easy to drive, especially if you have some rear toe in. Depending on the type of set it can almost be too understeering when accelerating. Experiment with clutch pack lsd to help mid-corner behaviour. Flatshifting doesn't damage the engine in FZR
XRR is a bit harder to drive, and hard to drive fast. The turbo lag makes it tricky and the rear really steps out quickly when you're accelerating out of a bumpy corner. I've always braked a bit early with XRR to get on the throttle well in time to have as much as boost (and speed, of course) as possible out of a corner. Your laptimes improve if you flatshift, but it damages the engine quite quickly.
Like all the cars, making the most of the available power requires very good throttle control. Smooth throttle regulation is good, weight transfer is less severe then.