They would never know you did it, so it couldn't void anything.
For what it's worth, this is a great thing to do in my opinion. I went to the track last weekend with my truck, it was well over 85 degrees with +80% humidity, that nasty wet hot feeling. The drive there my temps got to 150ish on the open road, let my truck cool off while the track was getting ready. Once the lanes opened I got about 12 runs down the track in about 90 minutes, I hot lapped it back to back many times. It took 4-5 passes for me to even breach 190 degrees and all the runs after that the temps stayed under 196 just beating on it. I left the track and the temps still above 190, within 10 miles it was back down to 165. There is no way it would have ever done that with the thermostat still installed.