Some will tell you its impossible to get a smps to work on breadboard, you and I are proof thats not true, allthough trouble of some kind is to be expected. A piece of copper clad board as a ground plane with star point grounding works well, I probably shouldnt say how many amps I've had through breadboard, suffice to say I couldnt get the parts out after.
Have you decoupled the chip?, a 10u and a 0.1 ceramic are a must, that can affect freq and stability.
Your efficiency is very good, more than expected, most of the losses will be in the switching transitions of the fet, the next will be copper loss in the choke, you can use a fet driver to reduce losses, but at that efficiency I wouldnt bother, the power the fet driver burns will probably worsen efficiency.
Yes, I dont know if its still in use however I maintain various pieces of industrial junk, new and old, one piece of kit had a merc arc rec in it, another had a tungar rec in it.