It's been said by Brunswick and others that the x-hole is used to fine tune a ball's reaction and that it does so by changing the RG diff.  Drilling into a certain part of the ball makes the core taller, increasing the differential.  Drilling into some other part of the ball makes the core shorter, lowering the differential.

Second, there was a rule change a few months ago lowering the maximum differential from 0.08 to 0.06.

Now, is there any data to show how much the differential is changed through an x-hole?  That is, take a ball, spin it to determine its true differential (there are a lot of things that affect it, like ball weight, pin distance, and top weight, so the "official" number may not be exact).  Drill it up in a "normal" way.  Don't make the fingers or thumb excessively deep or shallow.  Put the CG in a position to require an x-hole but don't actually put it in.  

Spin it again to find the differential.  Is it still a "legal" RG diff?  If the original differential was reasonable (say, 0.04 to 0.05 like most mid-range balls), how did the addition of finger holes change it?

Now put the x-hole in, and put it in a flare-increasing position.  Find the diff again.  Is it really that much higher?  Could the addition of a flare-increasing x-hole raise it above the 0.06 cutoff?  If so, I can see the USBC's problem with x-holes.

What about if the hole is put in a flare-decreasing position?  How much does it lower the diff?

SH