Martin, how do you find the Supremes on a RST? Mine came with Schwalbe Marathon Classic’s on and I haven’t changed them yet.
Despite being bigger (rated size 42x559) and slightly heavier, On my Raven Sport Tour the Supremes are at least as fast as the 35 mm Kojaks they replaced on good tarmac roads and are much better (improved comfort and grip) on rougher roads and occasional forays on tracks and paths. The 42x559 Supremes are probably about the largest tyre that you can fit on an RST while still keeping a reasonable amount of mudguard clearance.
I can directly compare Schwalbe Marathon Classics in 44x584 size with Schwalbe Supremes in 42x584 size, as I have used both these tyres on my old 650B utility bike. Both these tyres are slightly smaller than their rated sizes. The Schwalbe Marathon Classics were already a great improvement on the previous tyres I had, the Supremes were even better, giving lower rolling resistance while keeping roughly the same level of comfort and puncture protection. The Supremes look more fragile than Marathon Classics and are much lighter in weight, but in practice (about 20,000 kms of riding Supremes in various sizes) I have not yet had any problems with them.
When fatter 50x584 Schwalbe Supremes become available with the "new" 27.5" size (same as the very old French 650B size) there was just enough room to fit these on my old 650B utility bike, I found 50x584 very slightly slower than the 42x584 but significantly more capable and comfortable off-road.
I then used the 42x584 Supremes as a replacement for the Marathon Classics on my wife's bike, which doesn't quite have enough clearance for the 50x584 size. For my wife, in addition to the gain in rolling resistance, the fairly substantial weight saving from changing from Marathon Classics to Supremes makes a bit of difference, the lighter bike is easier to handle when pushing on very steep hills or simply when getting the bike out of the garage.
The only downside I can see is that Supremes are quite a bit more expensive than Marathon Classics, probably not too much of an issue as they seem to last for a long time.