That is a pretty common burner for home brew I think. I've been using one for years for my 5.5 gal batches. In fact I just added another one to my new brew stand to heat my water in a 15 gal keg. It took me about an hour to get from ground temp to 164, but I wasn't pushing it as it was my first time being the keg and stand.
All that to say, no, it's not overkill in my opinion. Plus, it takes a lot of heat to boil that much wort. Better to have too much than too little.