Easy analysis...
custom costs 300, so 3 boosts, or 15 SP. Would you spend 15 SP for +1 in each? 5SP on each individually? Only worth it if you're up toward 10 in each, to me.
Of course, that is a silly analysis if you boost, have +6, etc., as that is 15 SP you couldn't get without the custom.... But it puts things in perspective for a lot of positions.
So if you're short on flex, I see no reason for a tree for K/P, shouldn't ever determine your team getting a W/L which is what this game is all about to me.