I love the 3-CIM shifter, but the ratio is terrible. 17 ft/s is a great high gear for 3 CIMs, and 5 ft/s is a great low gear for 3 CIMs, but speeds of 9-12 fps low or high gear don't benefit much from the extra CIM. In order to see a noticeable increase in power/acceleration with the extra CIM, your high gear needs to be pretty high (I'd say 17 ft/s is a good starting point), and your low gear needs to be pretty low (4-5 ft/s is what I'd personally do with 3 CIMs).

tl;dr - the extra CIM isn't going to help unless your high gear is really high, and your low gear is really low.