To mikeb's point, there is a clear difference in where the vibes fall with IMD +40s in mine now, replacing original-spec flat-tops.
I didn't weigh them (should have!), but I'm not bothered enough to pull the thing down to re-balance it.
If the original factor was what, about 55%?, and if the difference is really as much as 9%+, then I'm still only about where I have some of my other motors. Anywhere not north of about 65% should be OK I'm guessing, for road use.
I'll know more when I've got more miles on and start using more throttle, but at about 500 miles only (due to nowhere much to go etc etc) I'm pretty happy with the pistons. The thing runs like a Swiss watch and I have yet to see even a whiff of smoke (even on first start up), nor used as much as a teaspoon's worth of oil.