No, it's not a "drop in." The radius is fine. But you'll quickly notice after mounting the Brown, that the frame has two big butt cheeks that have to be ground down and matched to the curves of the new safety. Personally, I prefer the Ed Brown grip safety to all others. But takes a great deal more time, patience and skill to fit the Brown than grip-safeties that only require a .250" radius. The initial radius is the same. It's those two butt cheeks that are a pain.
I prefer the Brown grip safety to the Kimber as well. I had one put on my TLE recently. Sent it to Kimber and they did it in their custom shop for a reasonable price. You can find pricing on their website. One good thing about sending it to Kimber is that they have a good understanding of the firing pin safety (activated by the grip safety) and can make sure that the timing is correct for proper operation.