This is more my un-experience : The R&P conversion forces you to lose the proper functioning of the control head, which seems like a dis-improvement to me. It also increases the steering ratio, meaning you have to turn the wheel farther to make the same turn. And unless carefully done, it messes up the suspension geometry even worse than it was to begin with (I have no idea how the Moss conversion is in this regard), aggravating the bump steer characteristics.
Although much maligned, I know from experience that the original Bishop steering box can work quite well if carefully rebuilt. IMO the main source of heavy steering is often overlooked : There has to be an absolute minimum of play between the rocker shaft and it's bushing. Used to be that replacement bushings were supplied slightly undersize, so you could ream them to match a worn shaft, but the last few I've bought were not. A friend of mine actually machined the box to take a second bushing above the original one, which I think is a great idea (and will try myself next time).