Lewis - I am tempted by the 1.6 supremes. I am sure they are faster than my current ones and would probably be perfect for the spotless bicycle paths of the country where I live. I just would like to do a longish test trip with my 2'' supremes, decently loaded with camping gear, before starting swapping tires.
I have 1.6 Supremes on my Raven Sport Tour, I use this bike for lightly-loaded day rides, mainly, but not exclusively, on good road surfaces. For my purposes, even on this sort of lightweight bike, the 1.6 Supremes are better than the 1.25 and 1.35 inch tyres I used previously, with no noticeable performance loss on long rides. I suspect that the effect of the increased comfort outweighs any possible extra rolling resistance.
I prefer the 2.0 Supremes on my touring and utility bikes. IMO the wider tyre copes better with heavy loads and unmetalled tracks or paths.
Not in the 26" size, but I expect the conclusion would be similar:
I went from 1.6 Supremes to 2.0 on my old 650B utility bike, the 1.6 Supremes went to my wife's 650B bike which doesn't have enough clearance for 2.0.
After the change I noticed a very slight performance drop and a significant increase in the ability to cope with mild off-road riding. The bike also felt more reassuring with a heavy shopping load. Comfort also increased slightly, but the 1.6 Supremes were already pretty good in that respect.
Before that I had Schwalbe Marathon 1.65 inch tyres on this bike. Both the widths of Supreme had better rolling resistance and comfort than the Marathons, which were already a vast improvement on the 1970's style Michelin tyres I previously had on this bike.
Possible downsides to Supremes are the very thin sidewalls, which could get damaged more easily than heavier tyres like the Marathon. Though so far I have done 28,000 kms on various Supremes without incident.
And the price, they are more expensive. But they seem to last a long time, my longest serving Supreme has done 11,000 kms so far and still has lots of tread.