must have missed that. was just curious where you saw that & if its really accurate, maybe their way of testing was different than what klipsch used to get the numbers... 6db is a pretty big difference.
Sounds like you've gone to great lengths to set everything up according to Dolby recommendations, which is great. As for the upgrade path, the center speaker is your most important, so I'd go with the best center you can afford. If it's the RP450C, then great. Next I would look at a second sub (and as big as you can afford), so R-115/SPL-150 or any other 15+ inch sub that fits your budget. Unless you have seriously lacking bass, then you might consider going second sub first and then center.
I don't have personal experience with your receiver, but it should be perfectly fine for these speakers.
As a matter of interest, why aren't you interested in Dolby Atmos. I found Dolby Atmos to be an incredibly worthwhile upgrade to my system.