PIS or point of initial slippage is the same as (but in reverse) as KIS or kiss point - the point of initial engagement. This is a distance measured in millimetres. The shorter the distance the quicker the (dis)engagement, as the clutch is travelling at a fixed speed over a shorter distance. The shorter the distance the more abrupt the (dis)engagement as you're reducing the 'dwell' time / distance for the clutch to engage fully. This is thought to reduce clutch wear, and I can follow the logic, though I haven't come across or heard of any measured comparison.
Going below the minimum of 4.7mm, though probably not recommended in official circles, seems to have proved okay to do. I have heard the same comments as Loz in relation to the F430.
The FD DBW module is a throttle remap. It changes the way the engine responds in relation to particular throttle inputs (itself a DBW system) through the throttle potentiometer / pedal. Anyone running the FD DBW (particularly in aggressive mode) will know that in first gear, the throttle is VERY sensitive under foot, this is because it has been re-programmed to give a much steeper acceleration curve compared to throttle input. Now at this point I have to say I haven't quite got to the bottom of the programming yet, but my understanding is that as a consequence of delivering acceleration from a shorter travel on the potentiometer, this in turn delivers the quicker gear shift only otherwise available at much higher revs - happy to be corrected on this point though.
Additionally I haven't got to the bottom of this yet; but my understanding is that on the pre MY'05 face-lift 4200's there is the setting for PIS / KIS, however on the later MY's and the GS, the PIS / KIS is also accompanied by a mechanism to constantly monitor and reset the PIS / KIS as the clutch wears, so effectively it resets itself to the chosen distance, whereas on the MY'04 4200's and earlier, the PIS / KIS has to be reset via the SD2 as the clutch wears. This is usually done at service.