I'd have thought that if the action is still tight (and it sounds from what you say that it's in new or near new condition) then the sights will be absolutely fine.

I think, for all practical purposes, there would be no quantifiable difference between the utility of a receiver sight and a cocking piece sight when used on a hunting rifle.

However, if the rifle in question is being used for long range target shooting, a receiver sight would usually have greater utility because, more often than not, it will come with a wider range of adjustment for windage and elevation - and in many instances will be much more easily adjusted.

That said, the actual mounting location of the sight (on the rear of the cocking-piece), should have little or no impact on it's general usefulness as a sight and indeed it could be said to be arguably better.