It might be worth your while playing with this [
http://www.badcock.net/] to get an idea of how much power a certain prop requires to get to
x rpm.
If you geared your 4900Kv motor at ~3.3:1 to get ~15000rpm, you could use a 7x7 to get 100mph pitch speed... but that needs 250W-in to do it.... so your present motor can't cut it.