The RX-8 is decent when it comes to the ride but not quite as good at the XFR. So far only Jaguar and Lotus have demonstrated the ability of having an exceptionally comfortable ride and sharp handling. The XFR has a proper luxury car ride, when you're driving around town it just glides over bumps.
I'm glad that the chassis was sorted on b-roads vs the 'Ring. It's fine to tune an all out performance car there but performance sedans need to be multi-talented and thus benefit from more real world tuning.
Speaking of the M5, I just thought it was fair to compare the benchmark with the latest performance sedan. Comparing the M5 to the XFR did remind me of when I compared the E46 M3 to the B7 RS4, the Bimmer felt like it was from a previous generation. I think it's fair to compare the M5 chassis to the XFR chassis but comparing the motors is somewhat unfair. I say somewhat because the M5 does have a V10 and the XFR has a supercharged V8. The character of both motors is different, the V10 loves to rev and has little torque, the SC V8 has loads of torque from the bottom but still loves to rev.
The only thing Jag needs to work on now is honing the stability control system, it cuts in way too early and the driver really shouldn't have to turn off the DSC completely to have a bit of fun. The M5 is clearly more playfull in this regard, I was able to put on a quarter turn of opposite lock with the DSC engaged. As far as the XFR comparison with the E55 is concerned, I think the engine comparison would be fair but the XFR would destroy it in ride and handling. So really each car in this class has different characteristics. I doubt that even the next generation of these cars will have similar characteristics, which of course would be great because each car would have something unique to offer.
I would love to try the Alpina B5 but unfortunately it's not sold here, maybe i'll give the B7 a try.