What's the reward for collecting all the T.W.I.T coins?

After you give Trowzer enough coins for him to have a BBQ, he'll ask you for coins one last time. This time he'll ask for 99 coins, which is all the coins remaining in the game. Before I make a point of collecting all the coins in the game I'd like to know if the reward is worthwhile, since I'm sure there'll be a handful of coins that'll prove a pain to get. I've tried searching online, but I haven't had any luck. Plenty of "this is how you get all the coins" guides, but can't seem to find any that actually mention what you get for doing it.

What's Trowzers final reward?

I collected all the T.W.I.T coins, and the reward was a fourth tonic slot. Which I was initially confused by, since that's the reward he gave you after the BBQ. Except, in reality, he just gave me a tonic which unlocked a fourth slot, but the tonic itself consumes a slot, so the tonic is actually just useless. Nothing more than 2,500 Quill gag from Playtonic.

I never actually used the fourth tonic slot tonic, but after collecting 200 coins and realizing what the gag was, I was able to confirm it via this achievements page.