I very done this many times to create colonies and frags faster than I could have otherwise. Here are a few methods I've used, and things to remember:
1) TANSTAAFL. Corals aren't 2d, they are 3d. When you do this with say an encrypting month you are really redirecting the growth to some place more convenient for you. For example back in DFW I have a mystic month that is a 8-9" circle. It encrusts at X inches per year at the edges, but is also getting thicker and growing little bumps and occasional shelves in the center. Think if it as constant volume, not constant linear expansion. When you make little frags the polyps can encrust outwards which increases their surface are quickly rather than forming a large lump which increases surface area slowly, but may still be producing skeleton at a similar rate.
2) TANSTAAFL Part 2. As soon as the pieces have grafted together you no longer have extra edges and are once more growing at normal pace.
3) TANSTAAFL Part 3. A large colony does grow a bit faster at the edge than a small one. I suspect this is because the land locked polyps are providing energy to the open edges. That same mystic monti encrusted several inches on the underside of a totally dark ledge and this is the only explanation I have. Note that this faster entrusting rate is still slower than the rate of you start a bunch of small frags ahead of the leading edge.
4) Practical Applications. My favorite is to glue a pea sized block of monti on a plug. Once it has encrusted I snap the original pea sized piece off and move it to another plug. The first plug now has a ring that is growing at the inside and outside edge, and fills in very quickly.
5) It may seem obvious, but remember that this will change colony shape. As an example I have a few colonies of the same purple stylo. One grew naturally, is tall, narrow, and has a really thick trunk to its tree shape. Another was started from 5+ little frags it is short, broad, and looks more like a hedge row with many trunks all interconnecting. I'm not saying one is better or worse than the other, but they are different. This can be used to your advantage if say you want a colony that curves along a ridge or other feature.