A bigger tire will NOT automatically give you higher speed. Whether it's electric or nitro. The larger tire has a signifigantly larger roll-out, as well as more mass. It will require additional torque to get the wheel moving, over a smaller sized tire. You will have to compensate for the additional roll-out by adjusting your gear ratio. By doing so, you will lose top speed. The rotating mass of the tire will also reduce to overall speed of the vehicle.

The other consideration is tire ballooning. The M2k's are very soft, and very large and prone to balloon under power. This will further reduce the efficiency of the tire, and reduce to overall top speed. As you said, physics don't lie. All a larger tire will do is overload your engine, and cause heating issues, and premature failure. If you want to go fast, you want smaller diameter tires, with a lower final drive ratio.

If you haven't changed your gear ratio's, I can tell you this: Yes your truck is faster. But it takes longer to accelerate, your runtime is probably only about 9 minutes, and your cylinder head temp is probably in excess of 240*f after a run. You can run it like that, but not for very long!