Tim, I've got an unopened X12 7.5 I'll sell you if you want more power than a 10.5 and less than a 6.5
. On the 10.5, I ran an X11 Stock-Spec one in my B4 for half a meeting, it was mint round the technical infield bits but like has been said on the back straight at Worksop it got passed by everything. My lap times were probably the most consistent they have ever been though and round somewhere like Broxtowe ie, small, I reckon it wouldn't be a bad idea.