I personally don't understand the necessity of going to angular contact bearings for the spindle. For one they are not a drop in fit. Then considering the maximum speed ratings for the bearings, the spindle bearings aren't the limiting factor anyway. The bottom bearing on the upper gear has the lowest speed rating of the 4 bearings for the spindle/spindle drive. I searched for the lowest speed ratings I could find on the stock bearing sizes used in the G0704, and found this:

Top of Gear: 6209ZZ bearing - 7500 rpm
Bottom of gear 6007-2rs bearing - 6800 rpm
Top of spindle: 32005 bearing - 8300 rpm
Bottom of spindle: 32007 bearing - 7000 rpm

The bottom bearing on the gear is the lowest I could find but is still at 6800 rpm and is a higher speed than a G0704, even with a belt drive, needs to normally run. So, unless running uber-high spindle speeds, I see no speed benefit.

So if it isn't a speed thing is it for improving runout? My own G0704 has very low runout from the factory. I don't remember the exact numbers, but remember measuring the spindle runout and being impressed with how good it was. Are others finding excessive runout with the stock bearings?

Is there another reason I am missing?