Yeah, break-in is something you have to factor in and not short-change with Gates, and I'd add, don't bother if you don't have an alignment tool. Polaris belts seem to be the most tolerant to poor alignment and clutch setup, and I suspect that's by design, considering how poorly set up some Poo clutches come from the factory.
Any belt will work better and last longer with proper primary-secondary alignment, side clearance, and deflection though. My mechanical sympathy makes me the wrong person to torture test most things, but I know guys who've repeatedly run up and down big hills trying to get the C12 to misbehave and couldn't. I tried Gates because the Poo belts are expensive, and several guys on here who ride the snot out of their sleds all winter had switched. They've worked great for me: only one I've hurt was my fault (jammed track), and it still held up the rest of the weekend.