I've had some pretty bad slips as well, lads at work have dealt with loads of alfine and nexus as we're a gazelle dealer, they reckon it's the shifter as it's not as accurate / well indexed as a rapid fire is and the hubs are very sensitive to cable pull..:
Have you not had any issues with slippage using the bar end? I'm contemplating switching to flat bar so I can use an alfine rapid fire instead.