I've seen XRT's on the bug servers lapping faster than I've ever seen an RB4 lap..still a bit early to see which cars the fastest, but your probably right, its not the FXO anymore.
Change is a good thing, having to relearn how to drive a car slightly, having closer competition between the class of cars..who cares if the FXO isn't the fastest anymore, its made the racing a LOT more fun now all 3 cars are being driven.
If you can lap the FXO at a 1.24.5 pace on BL1, clearly its not that much slower, laptimes on the bug test server rarely go below that level.
In my testing over the last few days, I've found the RB4 fastest personally, but have lost many many races to XRT's running better laptimes.