If I recall my high school chemistry stuff, the boil off rate would be set by the heat apllied and the surface area. So, same burner, same pot, same rate. If 1 gal/hr for a 5 g batch, 1gal/hr for 10 gallons.
Of course, if you need to sparge more to get the sugars, you'll need to boil more.
So far, I've had more experience thinking than I've had brewing....you don't think they are mutually exclusive, do you?
72 batches so far,
48 wine, mostly Loquat, peach, plum, prickly pear
23 beers and ciders
1 Tequila, from a prickly pear wine experiment that didn't work. I call it "Prickly Heat"