My kitty (she's younger than yours, 4 years old) has a heart murmur. I did a dental on her a few years back and the vet recommended I schedule a cardiac ultrasound on her before I proceed with the dental to make sure she'll be safe for the anesthesia. I opted not to do the ultrasound because, 1) it was a mild heart murmur and she's a healthy cat otherwise, 2) It was a bit out of my budget at the time, and 3) most kitty heart murmurs are harmless. So I weighed the odds, the doctor but a catheter in her "just in case" anything happened during the procedure, and she was fine.
Just over Christmas she contracted an Upper Respiratory Infection (was "sneezy" like you mentioned, and lethargic) and I took her in. When the vet was listening to her, the murmur sounded worse.
She has since recovered from the URI and I'll probably take her back in to have her doc take another listen to the ol' ticker to make sure it was just the illness making her heart murmur worse ((which I think is the case)). If it's still bad, I'll probably do the ultrasound on her
With an older kitty, however, more caution has to be excercised. The vet would probably recommend an ultrasound ((If they haven't already)) and most likely extensive pre-anesthetic bloodwork to make sure Sid's organs are in tip-top shape to handle the procedure. It's probably going to be a bit pricey.
But you want to take care of those teeth because they can start to affect different organ systems and really make him sick if left untreated!
((Hope this helped!))
Good luck, I'm sure he'll do fine. Keep us posted!