I don't think the Waterboy XP works anymore, on the Custom Courses,
The 15 Day Booster Bottle at 5% is about the best value.
Sporting XP enhanced Apparel in the Coin Box games also helps abit.
The Plus Memberships is worth the wait (if saving Credits by watching vids), by skimping (Blank balls loaded) - if you can put in the time to maximise it.