I have S3 heated seats retro-fitted into my 58 plate. Even at level '6' they don't get too hot. If I remember correctly, when Ash had VCDS plugged in, they rose to just over 30 degrees, which when shielded by leather, aren't that hot. Still very nice on those cold wintery mornings though. A very worthwhile mod.
As Ash says (he helped me do mine) retro fitting heated seats is the cheapest option. it's actually cheaper than you think. If you already have leather, they will sell for a decent price to offset the cost of the replacements with heating elements. I swapped my half leather S Lines for heated full leather S3 seats and after I'd sold my old ones, I'd only spent £50 swapping them out. I fitted them myself as it's very easy.
Bear in mind you will also need a new climate control with heating controls and a wiring loom, oh and someone to do the wiring unless you're confident yourself. I can certainly recommend Ash as he did a top job wiring up mine.
I sourced the climate unit from the same S3 the donor seats came from and sold my old unit for the same price. Free upgrade
The Kufatec wiring loom is a bit pricey for just a few bits of wire and plugs, but worth it for that nice "I've just wet myself" warm feeling that only heated seats, or actually ******* yourself can provide.
Fitting heated seats also cured a whining noise I was experiencing. Basically my missus complaining that my leather seats were too cold
