I've been saying for the past 2 weeks don't put any stock in Plummer's performance until week 6. Just like you shouldn't look at the trouncing the Cowboys got and say that Parcell's is never going to be great there, you can't put too much stock in Plummer's first game in the new system. Preseason doesn't count- if you recall, Gannon and the raiders were the league's worst offense in preseason, and the Bengals were the best team.
Anyway, Shannahan and Gruden are perhaps the best playcalling coaches in the NFL right now. As a result, though, they both have hideously complex offenses. Shannahan's playbook is jokingly referred to as being 3 feet thick. Plummer will take a bit of time to get used to Denver's routes, its plays, and all of its subtleties.
I think the biggest cause for hope is looking at his recievers. By far his leading reciever was Portis, with the only denver WR to catch a single ball being Rod Smith. Sharpe also had a couple grabs. Portis and Sharpe run the most predictable routes and are the easiest dump-offs, which accounts for their grabs. Once Plummer realizes he has 3 WRs to throw to as well and starts to locate those three, the Broncos will start to stretch the field vertically as well as horizontally and he'll start reeling in 300 yard days. He'll still be plagued by a bit of an INT problem, but no worse than possessed by say, Manning, or Culpepper.
Just my take on matters, but look how spot on I was about Denver's huge sleeper Defense. By the way, it should go without saying that, as big of a sleeper its D is, you are NOT going to want to start it against Oakland or Kansas City.
There you go. Take it to the bank.