The best I've ever got Sterno to do was to boil 8 oz. in 18 minutes. I still use it to warm food and to warm myself, but specifically for boiling water I made an alcohol stove:

http://www.zenstoves.net/

It's the simplest one--essentially a cat food can with holes drilled in it. Works great. Gives me a rolling boil in 4 1/2 minutes on 1/2 oz. of alcohol.

Regards, Vince

Edit: looks like they moved the one I built. It's called the Super cat stove. You can find it here:
http://jwbasecamp.com/Articles/SuperCat/HTML/Super_Cat7.htm