The SSU kernel relies only on the irq, so no, it doesn't poll. The DT continues polling at this rate even if the screen is locked or turned off (the poll isn't actually for touchscreen data but just a check to see if there is data available). While I haven't specifically tested it, I think that the touchscreen is never disabled, and the data is simply discarded at a higher layer when the screen is off.

Anyway, I can make a test kernel with tunable polling, then we can meassure if it has any impact by leaving the device alone for some hours with and without polling after a full charge.

Sure!
/sys/devices/platform/dsp/state is 0 if the dsp is not active.
Having such a toggle would be great, I was about to ask you the same. Beware that the op_dsp can only be changed if the dsp is not active, otherwise it throws a device busy error. I'll see if I can modify the op_dsp patch to allow changes on the fly, since the current op state can be modified by writting to /sys/power/op_active even with the dsp active.