Hot water seems to make it more potent, but it does taste worse. But that's not the worst part.
Hot water will make the starch in kava start to gel, so it gets THICK. Same effect as adding corn starch to gravy. But, that won't happen until you get closer to boiling.
Horrible tasting thick kava. Can't drink it -- just makes me gag. Ugh.
If you keep the temp low enough you should get better extraction but no thickness.
Me, I just use room temp water and knead, knead, knead. Works good enough for me. 2nd squeeze to get more 'lactones out rather
than heat anything up.