My older son is a 6'4" LHP and during his high school days, 2-3 different pitching instructors/coaches told him they thought he'd throw 4-5 mph harder if he dropped down a little (he was almost a straight overhand). He was low 80's his first year in college and when his college pitching coach made the same suggestion, he decided to try it.
Hindsight is 20/20 but here's how it looks now:
Pros: he does have more velocity, throwing 85-86 now, maybe even 87 at times, seems to get more movement, and his ball is a little harder to pick up coming in. Arm slot looks fluid and comfortable, but it took a while, a long while.
Cons: He was always a control-type pitcher, mixing speeds constantly and hitting his spots, but it has taken a full year to get the muscle memory developed at the new slot and his control suffered. It has gotten better but still needs work. His bread and butter pitch is the changeup, and it took almost a year to get comfortable with it from the new slot. It took a long while before the fluidity was back to it's pre-change status.
My son tells me constantly: "Velocity doesn't get college hitters out; they hit everything that's straight and between 82-93. Movement and location get college hitters out."
He still agrees with the change, but not everyone watching him pitch knows the history and it has been frustrating at times when he has struggled with his control. He rarely throws a 4 seamer, instead prefering the slower but better moving two seamer.
Hope this helps.