If you’re a seat-in rider, you’d want to lower your saddle as you can tuck the saddle in much more, and jump higher that way. Most seat-in riders have their saddle pretty low, it also saves your balls from getting smashed if you’re doing big drops.
Jumping higher isn’t so much a matter of crouching as it is compressing your tire before the hop, then tucking your body in mid-air. The crouch is what you do to absorb the landing, and having a lower seat gives you more room to crouch. Lowering your seat might help a little with your jump height, but it’s going to teach you the wrong technique and you’ll only be able to jump so high…
How are your rolling hops? If you work on rolling hops a bit, you’ll get a good feeling for the body tuck and see how it gets the wheel off the ground… apply this tuck to your seat-in hops and you’ll notice your height increases. I’ve found rolling hop height doesn’t exactly correlate with seat height, so you can work on that skill regardless of what you do with your seat.
Here’s food for thought: why mess around with seat-in if you’re concerned about higher hops? You said you already have a little aptitude with SIF but you haven’t practiced… why not practice that more? I’ll admit switching from seat-in to SIF riding is the second hardest thing I’ve ever done on a unicycle (the first being the ever-elusive wheelwalk), but it’s so worth the time and effort. You will EASILY double your hop height going SIF. I’ll give you a money-back guarantee on that
It can’t hurt to try lowering the seat (ok you might hurt yourself but really you know what I mean). I am constantly playing with the seat height. One day it will be so high I’m almost bottoming out, and another my seatpost is mere mm from the tire and I feel like I’m riding a 12". Just experiment with the seat height. There is no perfect formula, its just about personal preference.