When P3 is set to 0 it seems like throttle at 100% results in 100% torque of the currently selected power assist mode. Not 100% of maximum torque.
When P3 is set to 1 then the throttle supplies 100% of maximum torque. The problem is it's almost like an on-off switch.
This is the same problem I'm having. The KT-based throttle is not 100% off/on, but it's relatively crude and unrefined. You need to really fine tune the thumb pushing to get it right. It will not even start any watts until you are 1/4 through the throttle's range, then there is some little sweet spot of say 25-50% throttle travel, where you can actually modulate SOME power, then after 50% throttle travel it's at the max watts. That's not very linear!
Also, if you are going up singletrack there is absolutely no way to throttle only smooth because your thumb is needed on the handlebar grip and not on the throttle. It's too awkward and you are being forced to concentrate on not stalling or spinning out instead of what you are supposed to be doing: looking at the technical features of the trail climb and adjusting the rest of your ride accordingly. That's why so many dudes out there prefer torque-sensor based controllers. I get it. Just trying to somehow save this setup before I give up on it.
Another issue is that if you set it up with P3 at 0 (throttle power is linked with the PAS level) even at PAS 4 there is not enough power to climb normally. For example, if a bike is nominal 500W and the PAS 1-5 levels are 150, 250, 350, 450, 850, then 450 watts should be enough and should climb almost anything up to 12% grade. But if you are trying to throttle only with what you 'think' is level 4 and 450 max watts, it's still not enough, even with C14 set to strong power 3. My C5 is usually 12 but right now I have it on 18 and PAS 4 is still too weak. No idea why level 4 would not climb better. If this could work, it would be a lot easier because you could match the throttle travel sweet spot with a specific watt range that's linked with PAS 1-5. But you can't.
I'll keep messing around with these parameters. My backup plan is that I have an SW900 display and what I think is a DC Moto Controller or equivalent from EBikeling (they have discontinued that controller), but
the throttle is a lot smoother. I can switch the controller, display, and actual throttle over and live with that. It may be easier than messing endlessly with the KT throttle. KT makes PAS smooth but I don't think they really care about the throttle or they would explain how to smooth it out in English. They certainly do not explain it.