without seeing a pic of it i could not say for sure, but i would imagine that the BSA being .001 smaller might be better for graphite. sometimes you can have a record breaker car and with a axle .001 over or under all speed is gone.
There are 3 differences...
1. The head on the BSA Xtreme is the size of the stock axles.
2. The groove on the BSA Xtremes is a tad wider.
3. The obvious diameter difference.
The 93G is around .004 faster but the BSA looks a lot more like the real thing.