Are the Supremes ok on tracks/towpaths?
Yes.
I put 2.0" Supremes on the converted mountain bike I used for my last long trip, and also have them on it's replacement Raven Tour.
They are excellent on good tarmac, with their nearly slick tread and flexible lightweight casing, which also makes them very comfortable. Not noticed any grip issues with the slick tread when using them on roads in wet weather.
Probably better than tyres with more tread on the sandy cycle-paths that I used for long stretches in France, as I reckon they didn't spray sand about as much.
To my surprise they also coped quite well with much rougher surfaces, including rock-strewn tracks in the Picos d'Europa and NW Portugal, though if I had known in advance that I would be going on that sort of surface I would probably have used something slightly tougher (Duremes) for peace of mind. The total weight of bike, rider and luggage by the time I got to the mountains was about 115 kg.
They also reasonably durable (not yet worn any out) and have fairly good puncture resistance, just two so far in about 9,000 kms.
I have also used the 1.6" version of the Supreme, in the 650B size, which is slightly larger diameter than 26". Much lighter than the similar-width Marathon ordinary I had on the bike before, and a bit more comfortable and easier running. Also coped well with towpaths and local tracks, though I haven't tested them on very rough tracks. They are now on my wife's bike after I found out that 2.0" Supremes would just fit on my 650B bike but not on hers, she appreciates the added comfort compared to the Marathons. Wife is small and light, so she can use much lower pressures than I can.
Direct comparison of 1.6" and 2.0" Supremes on the same bike - the 1.6" are a bit lighter, feel very slightly faster (no figures to prove this), noticeably less capable on very sandy tracks and (for a fairly heavy rider) less comfortable than 2.0"
Downsides of Supremes are the price, and the sidewalls are thin, so probably more vulnerable to freak accidents than something tougher like Marathon ordinary (for example unrepairable gashes caused by large pieces of broken glass or sharp metal debris).
