If you have an ohm meter, ohm your current probe/sensor. If it reads approx 10K ohms @ 77*F it's a resistive probe. You can experiment with the temps and resistance readings. A thermocouple probe is much more accurate. It's used in the Thermapen for instance. To learn more about it go to www.thermoworks.com
But Hey! This is BBQ, not rocket science. ;D
To answer your question, you should not loose more than +/- 5*F by adding an extension. I've also noticed the Maverick male plugs seem to be a tad longer than a standard 2.55 mm jack. I think it is by design.