From what i hear they are about the same, perhaps a few degrees better. I think the main advantage is they cope better with sli systems where the ac silencer would have its airflow almost blocked. There are a few reviews around comparing the two, google it. I remember reading one but i can't remember exactly what the outcome was...
Gpus can go over 100 before they even throttle so i would say those temps are fine. If you currently have an ac silencer, i wouldn't say its worth getting an accelero.
One problem i have with the ac silencers is the lack of airflow over the voltage regulators on the card. I have a 60mm fan strapped to the back of mine and it works wonders. Cools various other components on the card that the silencer doesn't. I learnt my lesson after the voltage regulators on an older card blew up because of poor airflow as a result of fitting an ac silencer.