The purpose of speeding up (or slowing down) the gear is not entirely left up to the synchro ring on it's own, but also the energising lugs and springs play their own part as well. What series gearbox are we talking about here? 1.6mm sounds fairly high for a 60 series, more like a 71 series, either that or they are like new.

Roughing the surface of the dog cone will certainly help but it will be short lived. It will polish out soon enough. Some cones have their own machined grooves that allow a place for oil to travel to allow a harder grip between the synchro and the cone. It offers a lessened surface area for oil at a reduced syncro life as the trade off. The problem you have may not lie with the synchro or cone itself but maybe with the energising lugs. As they get older they wear the square shoulders off the tab and offer less resistance to the selector ring as it applies the pressure to the energising lug to the synchro. The energising lugs release earlier while the gear is still being selected and it can cause some short lived grinding as a result of the gear speeds not being matched correctly.

It's a tough one to call over the phone but the oil is the easiest way to try and sort it out, before pulling the box apart.