Are you actually trying to do 20 gallon batches now, or are you just trying to think ahead before brewing a rig? I'm building my rig to be able to handle anywhere from 15 gallon pots all the way to 30 gallon...
I just swapped out my little crappy burners for two of these bad-Larrys... they're freakin' fantastic. I've only done a 10 gallon batch since these burners have been put in but I would have NO reservations at all on doing a 20 gallon batch, which my rig is set up for.
Plus they're really cheap!
one note... I went natural gas (plumbed a line out to my garage from my house).