I would be easier to help you if you gave us the full reduction of the gearbox and the diameter of the pulley that spools whatever line, strap, etc. is doing the winching.
With our current design, we have one CIM, a 10.71:1 reduction and a .5" diameter pulley (the shaft itself) which seems to perform admirably, though we could probably work it around to climb even faster if need be.
A wonderful resource which you should take advantage of is JVN's mechanical design calculator (
http://www.chiefdelphi.com/media/papers/3188). In the linear mechanism tab, you can set motor, gearbox, pulley, and distance traveled parameters and get a basis estimate of what you should expect from your system. I'd put what you have currently in and see if the calculated numbers are in line with your expectations. If not, play around with stuff you can change like gearbox rations and pulley diameter to see what would work best for your team's resources.