One of WJB's mates was using warmers run from a genny at Donnington last year, and the genny was only just providing enough power to take the chill off the tyres - certainly nowhere near the temperature you get the tyres up to after a few laps (as measured by the Jools forefinger).
Before y'all go in for warmers and renting/buying gennys, you might want to check the warmers power consumption