Get it and boost if you're going to get it anyway. Do all the side missions first if you have any left because you wouldn't be able to do them after boosting. There's no achievements that I know of between WT2-5 so you won't miss anything. The only thing you might not like is that you will have a permanent boosted character label on your character on the selection screen.
Up to you really. It'll save you a bit of world tier grind but it doesn't take that long to get to WT5 anyway. BUT boosting maxes the first 3 specializations, which does save a lot of grind.