Sidious and Plagueis are both much faster than any of these Jedi, as well as better duelists than any of them save for maybe Windu. Maul is a better duelist than any of them as well, again save for Windu. Plus, they're power in the Force trumps any of the Jedi's own. It's by no means a stomp as I may have made it sound though, it will be difficult; but I think the Sith have it.
I do believe people are underestimating numbers. Windu + a mere 3 people less powerful than Dooku, Qui-Gon, or Yaddle just about beat RotS Sidious.
Having Maul is *very* helpful because 5 apiece, with Dooku and Windu as cornerstones? That is not going well even with two Sith as powerful as these.
Maul means he can keep 2, maybe 3 busy (depending on the individual). Which drops down the numbers each of the big two to a reasonable number. I'd say this is a pretty good fight, with near side having an overwhelming edge, both capable of winning.
Remember, just because a force user can blitz some lesser Jedi, doesn't mean they aren't using up energy *and* giving stronger foes room to act powerfully when doing so.
Hm, how much does that downgrade them...? I don't think we know *anything* about Kit Fisto in that timeframe...
Well, it reducing Aayla to a Padawan, so she goes from 'somewhat helpful in keeping Maul busy,' to 'really not going to help.'
Most of the others were older and already had their positions in that time to begin with, so probably not much change. But even just the reduction of two warriors seals the deal, since it means they'd need to draw off better fighters to hold off Maul, and they can't afford to spare that.