yeah, I'm sure it'll be fine with the stock pinion. I'm not sure if AE supplies a different pinion for the RC18MT vs. the RC18T but I can't be positive. Just try to keep on eye on how hot your motor gets.
actually the reverse it true grg. Since you'll be running a taller tire the truck will have a higher top speed. This is because the "rollout" is longer. What that means is that one revolution of the MT tire will move the truck a little further than one revolution of the smaller tire because the MT tire has a larger circumference (distance around the perimeter). So at any given motor speed the truck will travel further with the MT tire. Obviously anytime your travel further within a given amount of time you are traveling faster. The downside is that you will lose a bit on the acceleration side of things. The truck won't be as punchy from the get go. The MT tire changes of overall gear ratio a tad so you lose a bit of torque, which is most noticeable at lower speeds. hope this helps..