longer run times?

[kneedeeppow]

i had asked some questions re 4s/5s/6s lipos in the general electric section but had the thread hijacked and got very little info out of it so i will try here. i am currently running a mmm v3,2200kv motor.powered by 5000mah 4s 30c lipo with 14-18t pinion 46t spur in my mbx5t. i currently get about 13 minutes running out of each pack. i am after 15+ minutes as we are starting to see 15 minute finals pop up at a few evemts here.
does any one have and suggestions? maybe decreasing motor size to 1600kv? will that be a noticeable drop in power and speed?