I've fired a few thousand R1M3 rounds which are more or less the same as VIIIz, and like it pretty well. Running out though, and they're not importing any more.
The Privi Partisan everyone likes so well used to be marked .303" Mk.8z as well.
Is it bad for the rifle? It may not do well if it's had a lot of Cordite Mk. VII run through it previously, but aside from that, no dramas. "YRMV"