I bought a couple 20 gal pots and didn't think about the fact that it seems pretty big for a 5 gallon batch, my preboil volume is usually 7.5 gallons, and I don't really see myself doing 10 gal batches any time soon.
It's fine. You will have a higher rate of boil off but I do the same thing and just do full boils with adjustments made for boil off. Haven't had any issues.
You'll be glad you bought that big kettle when you do a high-gravity beer with 20-plus pounds of grain. And never say never...you might do a 10 gal batch some day.
If you decide to add a valve and thermo, mount them side-by-side, so the thermo isn't 8 gallons up on the kettle.
If you still think it's too large, I can give you my address to send it to. I'm looking for a 20.