Update on my brakes since I spent a few days at KOH, did a couple rock trails and probably 100+ miles in the dirt (NOT RACING). Also some additional specs and stuff I measured that Hydrodynamics had asked:
-Pedal ratio 5:1, might be closer to 6:1
-Stainless steel brake lines the ENTIRE way, no hardline whatsoever
-Honda Accord brake master
-99-04 Super duty axles with stock type brakes, as I recall it's the powerstop kit with rotors/pads/calipers, but the calipers I believe are just factory and painted
The brakes are extremely good. And the ibooster being electric has perks I didn't think I'd use right away. At one point I was a winch point without the vehicle running, and I was able to keep the tires locked up with just the ignition on and it was easy and could do it all day. Could maybe do that with a vacuum setup once or twice, but I could get on the brakes and let off (we were trying to get the buggy fixed since a fuse blew) all day.
Bleeding I've done with the booster on and off. My setup with the booster on feels "squishy" because of how de-sensitive the brakes feel due to how much assist is going on. If you turn the booster off you can push the brake pedal halfway or so and it's a good bit of effort, you turn it on and you can bottom it out with your hand pretty easy. I'll bleed them with the booster on, then turn it off to feel the pedal and see if it's squishy. Part of that could be from the stainless lines swelling.
My biggest complaint is pedal feel. At one point I had my foot on the brake (left foot braking setup) and didn't even realize how much braking was going on until I stopped and could smell how hot the brakes were. I can lock up the 40s and have not been worried at all about the brake performance. But the pedal (WITH MY SETUP) does not give a lot of feedback between barely pressing it and max braking.
The pedal ratio I think is on the higher side, and I have stainless brake lines, and it's a small master cylinder. I think any/all of those could be changed to upgrade the pedal feel, which really is just how much force does it take to get a reaction. Again, the pedal is squishy, but the brakes aren't squishy, there is just a lot more assist going on than I anticipated.
I have an ibooster on my FJ40 and my Hilux now, both of which are 70s vehicles with a pretty large motion ratio at the pedal. Both of those will lock up the brakes like there's no tomorrow. The FJ specifically is worse than the buggy I think, there is hardly any feedback from the pedal and you don't have to push it far to lock them up. You go to slam on the brakes, and really you just need to blow on the pedal to do what you want. This is a setup issue, not a booster issue. Just something to tune in. I would 100% have no issue running the ibooster and Honda Accord master on 99-04 super duty brakes right out of the box. The master seems to flow enough to do everything needed, but small master = less force from your leg, plus you have a ton of assist from the ibooster, so without a lower pedal ratio it might take some getting used to. Kind of like a vehicle with a soft throttle pedal versus one with a huge return spring.