Ummm...any winch that can pull it's rated load, 8k, 9k, 10k, etc, can easily pull twice it's rating with a snatch block. Three times it's rating with two snatch blocks.
IMO that "rule of thumb" is based on people assuming you should just use a single line pull.
I'd argue that you're better off saving some money on not going crazy on winch overkill, and put the saved money towards more rigging, which you're going to want anyway, and which will allow you greater flexibility in types of recovery you can perform.
Of course, if money isn't an issue, there's no harm in getting a winch that's more than you need, depending on the weight premium.