Would a driver with 12 degree loft be too much for a swing speed above 100mph?

Swing speed is only really applicable to the shaft flex. 100 MPH would be perfect for a stiff shaft, maybe even some X stiff shafts. The key thing to look at is launch angle and spin rate. With a stiff shaft, your swing speed and a 12 degree driver the ball will be coming off the face very very high and spinning like mad, which will rob you of distance and consistency. I would recommend a 9 or 9.5 degree for you with a stiff shaft. The best thing to do is go to a good Golf shop and get custom fitted, they will find the best driver and shaft for you and your game.