All I can say is that I have never experienced valve seat problems with unleaded fuel with any of the old motorcycles I have ridden since leaded fuel was phased out, including iron-headed singles and twins. Many British machines were designed to run as well as possible on low grade fuel.
The only significant changes I have noticed with unleaded fuel of the kind sold at pumps across the EU for the past decade or more are slightly hotter running, a nasty sensation on the skin when one washes one's hands in petrol after a roadside repair and a marked absence of sparrows and other small birds from urban centres, suggesting that they know how much more harmful to the health this stuff is than the old stuff.
I did have problems with Niton-tipped carburettor float needles in Amals and Bings because the additives in 95-grade unleaded made them sticky. A change to 98-grade cured this. However, I gather that 98-grade is due to disappear. Another irritation is the tendency of unleaded to "go off" much faster than the old leaded fuel. I have also found it necessary to remove water from float bowls far more frequently than previously, although this may be due to filling stations economising by not cleaning out their storage tanks as often as they used to.
In summary, I would say that I believe that my motorbikes used to run more sweetly on leaded fuel but that they run satisfactorily on unleaded 98, including the ones with tuned motors, as long as one retards the ignition by a couple of degrees or so. But I have never had a problem with valve seats and am inclined to view the whole valve seat recession thing as a bit of a scam. If it ain't broke, don't fix it. Having a replacement valve seat come loose at speed - or even at a standstill - isn't much fun.
Paddy Keating