Getting 9 minutes of runtime form an 1/8 car is average to pretty darn good. Most racers pit between 6-7.5 minutes in races. I'm not famaliar with the Spektrum temp sensor type but the ones in the past from other manufacturers that you wedged a sensor between the engine heatsink fins never were as accurate as an infared gun. If the smoke is good, engine sounds/feels/drives great then you're good, don't stress over engine too much otherwise striving for a pre-determined number can hurt your engine in the long run.