Thanks Steve, its good to get advice from some one who has used them.

What price have you been quoted?
Ive not looked at the super scary high precision ones yet but standard A/C are about £15each and then the matched Pairs are about £40 each, where as I can get 5 of the deep groove ones for £35,

Did you notice if the balls in the smaller external dia. are smaller? Reduced mass seems to be one of the critical points for High speed so would be a bonus for the deep groove ones, as I could use 4 or 5 to regain the strengh

they do get a little warm when running at full speed but it does not seem to be causing any issues
out of interest are the push fit or a tight interference?