With $5000 for the theater, yes I would make some changes:
If you have an acoustically transparent screen (Hopefully you do) then get three of these across the front: Chane Theater Ten
These are high efficiency theater speakers that will offer great dynamics and get you reference level sound (at a reasonable distance).
For the surrounds, four of these JBL 8320
. They are THX certified and the angled design helps get them pointed at the listeners, good bang for your buck.
With $3000 left, I would focus on a pair of excellent subs, that Denon is still a good option here. Since these are all highly efficient speakers, you won't need a lot of power or external amps to make them loud. You could go with a nicer receiver but the sound quality will not change much (unless you go with Audyssey MultEQ XT 32). Better subs will make more of a difference; I would recommend a pair of the Rythmik FV15HP
. These are very impressive subs, you will get down to 14Hz extension, perhaps more depending on your room, and this will provide you with bass you can feel, which really adds a whole new dimension to movie viewing. Their high output will be perfect for your medium/large room, and having the pair means bass is more consistent throughout the room.