The priority calculation you suggest would be a considerable improvement over the current system of first-come-first-served. If it were implemented then I think it's likely that the incentives outlined in my post would cease to operate and that average transaction sizes would not increase significantly.
There may be some incentive to break up large change if it is believe that spending it is not likely to be required in the near future, in order to ensure that coin holdings of adequate power always exist. It's hard to do this correctly as it requires the forcasting of both priority and spending far in advance.
If there were many contending transactions there would be a tendancy (all else being equal) to retard blocks with multiple ins because scriptSigs are so much larger than scriptPubKeys. I haven't an opinion whether this is bad or not.
I think there may be some small distortions to the value of older transactions on the grounds that they are so powerful. People might be motivated to "exchange" the private key instead of performing a transaction in order to maintain the power of these old transactions while still "spending" them without actually spending them. This would be nothing but advantage to bitcoin as these key exchages don't show up in the block chain.
Also unfortunately, there would be the incentive for the following undesirable behaviour:
Suppose someone has just been paid a large sum of bitcoin (1000BTC) and needs to make some small payments(1BTC). They also have recently been paid some smaller amouts of bitcoin(0.5BTC). If they try to pay the 1BTC debts using their 0.5BTC credits then their transactions have very low priority. This transaction can be represented as 2 * 0.5 BTC = 1BTC
If however they include the 1000BTC then the transaction has a much higher priority and it's not really wasting any significant accumulated age-derived power of the recently received 1000BTC. This transaction can be represented as 1000BTC + 2 * 0.5BTC = 1BTC + 1000BTC. This would approximately double the size of this sort of transaction.
But please, create your own client and try to break things on the test network!
No incentive to do so thanks!
ByteCoin