Interesting topic.
I've got into some funny habits to cope with being underfinned with deltas at Budgy. I've used a 20cm on a 75 wide board and a 7.8m. Couldn't put any pressure on it ( I have a low spinout tolerance.. a couple of spinouts and you need a swear jar and I'm over it)

I just used minimum pressure on the back foot and dipped the leeward rail. Works best overpowered as I also tip the rig towards the eye of the wind and ease it out a bit. I can shoot upwind with that but don't go as well underpowered or normally powered using that technique..I'm not the fastest sailor upwind but I get there.
I also sail with the backfoot out ot the strap a fair bit when not powered up a lot although i don't know if that's related.
I just bought a 22cm delta XT for that board and wow what a difference! I can push on it like a normal fin and sail more normally.
I also use a 19 delta with the 90ltre and 7m sail so I really need to get a larger one for that eventually.Much less effort and more fun factor!

I started speed sailing at Budgy on waveboards ..now they were a pain to get upwind. I used to think there was problems with the sailor but the first day i got on the slalom board wow! So easy!