If you are carrying a sub-compact gun, presumably it is because a compact or full size gun is too large for your application. Adding a finger-grip base plate will in crease the size of the pistol in the most difficult dimension to conceal: the grip length. Adding a base plate to a Glock-26 gets you a Glock-19 with a shorter barrel and sight radius AND lower ammunition capacity.
Probably one of the smarter gun modifications I have seen was a Glock-17 with a grip cut down to Glock-26 size and carried with a standard Glock-26 magazine, and Glock-17 reload. This gives the concealabilty of the smaller gun, but the accuracy of the full size.