No additional incentive is needed, although the thought is greatly appreciated. I doubt she'd be insulted, rather she'd be very touched that you think that much of her. If you insist, you can certainly reward her with the "difference" in commission... but honestly, I don't think it's necessary. We are nicely compensated for what we do.
Additionally, I seriously doubt that you'll find a property listed today, that'll accept an 83% offer... so the likelihood that your agent will receive $2,500 less, is pretty slim. If the seller were prepared to accept an 83% offer... they'd probably be priced closer to that number... and if they were going to accept 83% of their list price, they'd probably attempt a couple of price reductions, trying to attract a higher buyer, before they'd be ready to take your $500,000.
The bigger issue is that your agent benefits from you being happy and satisfied with your home purchase.
The agent should be your trusted advisor, and is there to work for you, on your behalf, helping you get the best price and terms on the home of your choice, no matter what the percentages come out to be for the agent. If you are pleased with your agents' representation of you, you will want to refer that agent to others. Those referrals will be what an agent values, much more than $2500.
If, however, you want to show your appreciation and are so generous, you can offer a bonus. I wouldn't do it to assure that the agent works in your best interest, though, that is his or her job.
You know who else may be insulted...the seller. I would not expect an offer of 500 on a list price of 600 to be received well. While it is a buyers market, one needs to be realistic.
Good luck with your search!