If they haven't they really should have. It's a great price for a truly important upgrade to any M. In my opinion and that of others (not all) it is a preferable upgrade to the polybush option, but if you are reading this and haven't done either may I suggest you buy these and get some brand new OEM rtabs (for £40 or so), sit back and marvel at the control you now have of the rear end.
Many indys will point you the polybush way as they take 2 seconds to install, for the OEM rtabs it's rather more difficult and ideally you'd use the specific BMW tool, but worth it imo. I had polybushes before (powerflex) and removed them shortly afterwards, really didn't get on with them.
Turner make the best versions out there of these limiters AFAIK, there are cheaper (plastic) ones available but not much less than this sale price.
GLWTS