Stovetop Boiling - how many BTU?

I'm in the market for a new NG range and would like to be able to brew on it. I know that some of you boil 6 or 6.5 gallons on your range with no problem, and i'm wondering how many BTU's it takes?

The 2 ranges that i'm looking (GE profile slide in range) at have power burners at 15000 btu and 18000.
My current one is 9000 btu which can bring 6 gallons up to a boil with the lid on (in about an hour from mashout temps), but won't hold it without the lid.

I have a viking range with 18500 btu burners. It takes me about 50 minutes to get 5 gallons to boil from tap temperatures

I have an Electrolux with an 18,000 btu and it still takes quite a while to bring tap water to a boil, so I would recommend as so all things equal I would opt for the bigger burner. I have no problems maintaining a boil.

50 min isn't bad to bring tap water up to a boil, i can find things to do while heating up the strike water like mill grain, math, etc.
Do you guys get a full rolling boil with the lid off? (that's my biggest problem right now)

SWMBO and I are about to upgrade our stove as well. The one she wants has a big center burner that has 17,300 BTU's. I'm guessing on really cold winter days when I don't want to go outside to brew I could pull off 5 gallon batches on this bad boy? Or should I push for something bigger?