Funny, we were only discussing heated seats at the last Curborough breakie meet. Some of the kits can be really effective, but whether as a replacement for a heater, I don't know.
My gut reaction is no, but in fairness, I've never tried them in a Westfield, AND I'm basing it on how I tend to feel the worst of the cold when Westfielding. Others who feel the cold differently, would quite reasonably have a different view...
(I find even if I'm really wrapped up, it's my feet that end up feeling the cold the worst, a heater has completely solved this. I don't know how heated seats would help, when trying to keep the extremities warm, though I can see them being a really handy addition to a heater!)
Heated seats seem quite a popular addition over in Caterham circles, though there again, I get the impression that it's more as an addition to a heater rather than a common replacement.