The point of collecting coins after all upgrades

Bear in mind that Temple Run 2 is a 'free to play' title - they make their money from people buying in-game items, so there are lots of ways to burn through coins and gems.

In this case you can 'gem-power' a power up so that the next time you use it you get more from the power. 1 gem gives you 1 boosted power up use, 2 gems gives you 2 and so on.

If you're going for a long score run you might cash in 5 gems so that you can use the boosted power 5 times.

To be honest, for the amount you get back it's a waste of gems.

After unlocks & upgrades, there is no real use for gold (unless you need to purchase characters (which has been completed)).

I guess it's just stackable.

Edit: The Head Start is a useful one to purchase before each game.