OK, I was wondering what that number was. I initially thought it was the combat bonus!
Hmm. I think maybe reduce the size some and increase the damage some, so they meet in the middle so to speak. Personally, I wouldn't worry about a great wyrm red dragon doing insane amounts of breathweapon damage. IMO, they should and the PCs need to find was to counter this (fire resistance, bonus to saving throws, etc.) or avoid the dragon or figure out another way to deal with it. However, my general stance is that most PCs should be a TPK against a great wyrm, so I am biased.
I think smaller size (and possibly less HP) and more dangerous attacks will make a fight with a dragon more memorable.
I am less worried about the breath weapon damage (as the breath is something that pc's are the most likely to have prepared for) and more the trample/tail sweep/wing buffet damage spike. Physical damage is much harder to resist than energy damage.
Your idea is pretty good, I will see what I can do to create a more effective balance.