Great to hear you are loving your 8'3

Fins? No, though I have tried it as a twinser taking the rear two out... I personally didn't like it. And I've not tried any other type of fins in it. I had an 8'3 for a while, but now have a 7'6 and the smaller the board the better it is for slashing about on.
You mentioned bigger cleaner waves, well I didn't think the 8'3 was that good in bigger head high stuff, certainly not those with a big belly in them.. the hollower the better so it could launch into them... that's what I found. I preferred riding chest high and smaller and yes the the crumble. And m sure you'll enjoy it, IMO the Shroom is one of the most exciting boards to ride.
If you get chance to try a smaller one, take it.. The 7'9 is a fabulous board, smaller tail and very slashy.
I've worn out the deck pad on my 7'6 at the front foot