My understanding of FIM is that it adversely affects growth much less than topping. I've done both, and I prefer FIM as the plant seems to only notice in positive way.
FIM can also be a matter of degree, not just black/white. If you leave any hormone producing bits in you're in the gray, ie not 100% effective.
The reason I like the blade I use, is the I come at the flower from 3-4 places (12 o'clock, 3 o'clock, 6 o'clock, 9 o'clock), outside edge, down into the center. Go around and repeat. That's the best way I've discovered for good removal and minimal damage to that outside cup. Scooping outside edge, into the center, then up to the opposite outside edge can be asking for problems. The risk is damaging the outside cup. You want to maintain that.
Good luck!