I prefer honey over cane sugar. Both ferment really clean though. I just think honey is safer to use and I'm not going to try boiling it first. Probably not true, but it's stuck in my brain and I'm sticking to it!
I'll usually add it after the krausen has fallen in the primary... or at 5 days after fermentation has begun, whichever is later. You want the yeast to clean up a bit first. Otherwise, they'll jump right on the simplest sugar and get lazy. Warm the honey container in hot water first to make it flow faster.
I would just stick to the recipe, then add a pound or two of honey later. It's an IPA! A little extra alcohol isn't gonna hurt! Lol!
As for a gravity of 20...
Many people hit but can't go under this threshold with extracts. A lot of it has to do with the unfermentables in extract. Technique, yeast viability/amount, and fermentation temp are also big factors. I'm sure you'll figure it out quickly.
Cheers, and good luck!!!