Discharge initiation and sheath formation are serious problems of high current (several MA) dynamical pinches because of the enormous power input. The fast 300 kV plasma focus SPEED 2 allowed studying the discharge behaviour in a wide range of power input (10-100 GW) previously not accessible. The main results are: (i) high power input causes fast sheath formation (<100 ns); (ii) there is an upper sheath energy density limit (<or approximately=100 J cm-2) that decreases with increasing insulator radius; (iii) this power and energy density limit is enhanced by surface conditioning either using high energy (>100 keV) photons with which the insulator surface is irradiated during discharge initiation and sheath formation or using insulators with coppered surface.