Obviously, start with getting the frame / components straight.
My first question would be what axle?
ASN Canada rules? 50″ max width? I’m guessing you have a 50mm axle? If that’s what you’re running, if you don’t have it in there already, I’d start by going with a soft or super soft axle to try to get that narrow rear end to flex as much as possible – and remove / detach the middle axle bearing. If you’ve got a medium/hard axle in there, it may lift quickly with a stiff rear for decent entry, but sit back down too soon on exit, especially with the combo of lower HP and lower weight and narrower width of the Canadian-rules LO206, compared to the Rok GP.
If I’m guessing correctly, the reason high air pressures are helping is that the sidewalls are no longer flexing under side loads, and instead of generating side bite, the tires are now just sliding which will help the kart rotate – the kart might not be binding as bad on exit, but you’re not getting the forward drive either. With lower pressures, I’d guess you get the side bite, and with the back end setting back down too fast, you’re probably having to add more steering input and killing your exit.
A soft axle is almost a “must” for the 4-cycles at 50″.