I'm using the latest 1.17 firmware, I redid the pid autotune few days ago with my e3d 24v 40W and S270
I also had errors with P0.4 and P0.3, in the end I redid it without any P param it worked ok (no more warning, temp control ok)
That seems weird, the documentation says that it defaults to 0.5 for 1.17 which should be worse. Almost sounds like a bug ?
You probably won't get more than 10C overshoot assuming you have the correct voltage heater for your power supply. I suggest you choose a target temperature that you commonly print at.
Thanks David, I'll give that a go when 1.18 moves to stable.