[old jupiter]

Frank is right that getting the cat lit off sooner doesn't effect fuel economy, just emissions (though that is desirable, too). If you want to get the cat hotter, sooner, you could try using a thermal wrap around the exhaust system upstream from the cat. You could test the effect before and after (with the engine completely cooled off between tests) using a stopwatch and one of those infre-red temperature "guns". The exhaust thermo-wrap might give your engine a small improvement in fuel efficiency and power, too, and a lot of racers use it. There are some limits to its use, though; you don't want to use it where it can get wet and rust out your exhaust pipe or headers.

Cd, if you are asking if one of the stick-on or magnetically-affixed heating pads could be applied directly to a cat, my guess is that none of them would survive the very high heat generated by the cat when it has lit-off.