Mine runs an 8 speed 11-28 with the same 48 front but unless its down hill i dont think i get much above 48x15 on the flat and mine rolls along 19 mph or so on the flat fully loaded for work .
You can get 7 speed freewheels with a 13-25 if you wanted a faster back gear .
https://www.sjscycles.co.uk/freewheels/ ... freewheel/
I use city jet tyres which roll better on the flat but might not be much use on more rough stuff although they can handle cycle / gravel tracks .
My upgrade was a drop bar conversion but it was mainly as for some reason my dodgy neck nerve means the angle of the arm aggravates it on a flat bar but not so much on a drop bar .It is now basically a hybridized CX bike .