I've met up with some S4's that can corner pretty darn well -- and the SRT-4 is the best handling FWD car i've ever driven -- I'm surprised the XF is in the same class as any of these 3 cars -- the s4 and wrx are both AWD with at least a 30 hp advanntage over the xf... and the srt 4 (with the LSD) is a beast as well..
I think in showroom stock trim, the xf is dead in the water against these cars.... I will very interested in hearing how things work out with the LSD 3.69 you're having installed. Have you gotten a price on that yet?