I think the energy star ratings are pretty reliable, except that from what I understand, they don't include things like ice-makers, which can use a lot of power - but that's probably not an issue in a deep freeze like you are looking at.
One question is - was your kill-a-watt reading made when the thermostat was broken? That could throw you way off. IME, a freezer will run about 50% on/off, so if you take the running watts and divide by two, you might be close - but that assumes the 50/50 cycle.
Our freezer is abut 30 YO, and I would put a new $25 part in it. I know it's been reliable for 30 years, not sure of the new ones. But mine is only using ~ $7/month now, so the payback would be pretty long.