So it seems Bonus XP has a cap. For regular level 96 mobs it definitely tops out at 3000 at around a 100 chain(I believe). For level 96 Champions it seems to fluctuate a bit but I think it caps at around 7.8k or so and then it's around 8.6k for the new level 99 rares in Nimgaul. They all seem to stay around this number after you break 100 chains and I've seen as far as a 1.2k chain without an increase beyond this point. I don't know if this is intended as a means of making it to where classes that can chain super effectively don't pull too far ahead of classes that can't or if it's not intended at all but I thought I'd ask about it.

100 chain is the cap. Diminishing returns have to kick in at some point!

Good to know