I think the key word in your statement is subjective. There are far way too many variables to have anything even close to a 'cap' to shoot for. Even on a per priest per raid basis, all sorts of unpredictable little things can throw off this 'value'. Aiming for some cap is generally not something I'd be willing to do b/c that extra mana is needed when something goes wrong. Under that circumstance do you call for a wipe b/c you don't have your target mana consumption or do you roll with the punches? Healing is more about dealing with the unexpected than following some flow chart or spreadsheet.
While I do have an issue with the concept of a 'cap', I'm not disagreeing with the above discussion that Holy will have a scaling issue. Holding everyone else to steady gains, and giving us an extra scaling regen mechanic is a double edged sword. We'll either be under mana powered or over, and the former is far more likely to survive MoP release. However, since the talent calculator is far from finished, I wouldn't just knee jerk. Give it time to formalize a bit more. It's far too early to analyze anything right now. At best, some loose musings about some of our new abilities are appropriate.