Equipment (clubs) are made available to you based on your level, not your tier. If you really just can't wait, and want to buy a XP boost, it would make the most sense to use the boost to push you over the level you need to get the new equipment you want. So I'd say toward the end of the previous level, and activate the boost just prior to your 1st round of the new WGT day to get maximum XP. If that makes sense.