In the NHL, before the lockout and the NHL salary cap, the Leafs outspent every other team.
Spending doesn't guarantee results.
Anyway there is zero chance that MLS will ever agree to such a thing as a salary tax instead of a cap.
Now what would be an excellent departure from parity would be removing all restrictions on signing academy players. Then teams will get out of their academies what they put in, and MLS' goal of developing N. American talent will come true. I would support that even though it will give a leg-up to Vancouver, because I want what's best for the league.