Yeah, dont expect more than 17 mins with those batteries. If you want more runtime just go with a XL motor. (in your case a 8xl would be good)

well i just got my first full run in on my 10xl, 9920 on 14 cells with 20/51 gearing and with the ib 4200s i got 15 SOLID minutes of running doing nothing but speed runs. not bad for the batteries first cycle...