Zero sign of it so far. I'm getting a very slight mineral build up that I just rub off with a scrubby pad. And I've been heating the mash, too, where the sugar density would be of course much higher. I think the surface area of the lid is far, far more than a low density heating element.
Before using that lid I was using a stainless dish from the pound shop, a real super flimsy one and I had to weigh it down a bit because it would move about. I wondered if I could improve the coupling using two lids together but the coupling went down, not up. And I used a steamer - the kind that stack and are part of a saucepan set- that worked really well, too, but of course you've basically got a saucepan in there at that point and I didn't want the handles to rub the sides of the fermenter, that's why I settled on that lid. It's absolutely pefect.
Basically you can turn any fermenter into a big old boil kettle for a quid and you don't have to go cutting no holes for elements and eletromecuting yourself AND I can completely cover the fermenter in insulation with bungee cords and it won't catch fire... like maybe it did when I used to do my 'bain marie' thing on the gas cooker.