Steve,
My experience with the mod is that I was loading .308 with enough powder to come past the shoulder, within 0.2 - 0.3" of the casemouth. After the mod, I had significantly less spillage such that it was less than the natural deviation of the powder measure, and going slow pretty much eliminates it.
I won't say it's totally gone, 'cause you can always get a load that's so close to the casemouth some will spill -- like a 90gr 9x19 filled with N320.
The indexing on the SDB seems more abrupt than the 650 due to the 650's mechanism- on the base, there is a ramp which engages an arm on the bottom of the shellplate which is rotated as the plate comes down. On the SDB, there's an arm that cams the plate around with high velocity. The rotation on the 650 is "linear" with the downward movement of the plate, while it's cammed on the SDB.
The 650 is much smoother after the mod. The detent strength affects the final decelleration as the plate locks into position.
-z