My 540 surpassed 60 ponies with ease. Your only option to go more compression is having 4 of them custom made, about 1000 bucks. Speaking of that, hold off on them plating your cylinder until you can send them a piston to specifically size it to your cylinder!
My KTM bill pipe showed a 2-4 HP increase over the hmf. Bill builds ones specifically for the 540s to breathe. Best bet would be an akro pipe but good luck finding one of those. I think one was for sale on Facebook the other day, there’s a KTM group and that’s where all the traffic went. The biggest power maker is the head setup. Good port job with +1 intakes and a multi angle valve job. I was fixing to deck my cylinder head but I was already about 60/80 thou with the cp 12.5:1 which is as close as these guys need to be getting. Your cam will matter big time too, 43.5 is a lot for a 540 and you’ll likely see bottom end loss, but an increase up top. Not really good for the XC engines, they like to spread the cranks at high rpms. If your pin isn’t welded and your axial clearance on the crank isn’t set to about .019 thousandths I recommend doing that, it’s almost a must. Your cane choice will come to be an 8/06 or an 8/33. I usually say the 8/33 is better but since your carb is so big you might want to compensate a little with the 8/06. Or maybe tell raceres edge your dilemma and have him go a little smaller on the porting to compensate. It’s all about finish that amount of air brought in that makes it happy. I also recommend a cooling mod with a bigger/high comp setup (NOT THE Y KIT!), a high flow water pump, and some gym time, cuz you’ll be holding on.
If the “o-ring” mod you’re referring is the o-ring on the accelerator pump arm, you wasted your time. Doesent do much.
The imulsion tube helps a little. It mainly helps at low RPMs to help get the fuel to a vapor. In theory helps get rid of bog, and since all the fuel is vaporized it can all burn, so better/cleaner burn, as well as efficiency.
Big reason stock engines with FCRs develop a bog is the leak jet is just way too rich. That’s why the adjustable ones work. I think the 540 is a little too big to benefit from those. Trying a current gen AP straight from Keihin this time around with some other things, we’ll see how it goes.