In my opinion, multiple accounts are a product of sandbagging. Address the issue of sandbagging and you solve both problems. I think that the reason people open multiple accounts is because they cannot compete at their current teir, partly because many others are playing below their "skill/teir" level.
The two solutions I've thought of are:
A) including match play win percentage in the formula to advance tiers
B) introduce player attributes, where as you move up to a new tier your actual player gets better (strength, precision, forgiveness, etc...). this way even though your player is hitting from closer tees as a pro, a legend would still be able to keep up are even hit farther because their attributes are better.
Two other things i think should be done:
A) go back to true averages and
B) count tournament/MPC earnings towards advancing tiers.