I'm about to get the boiler moved from the kitchen to an upstairs
cupboard,
but wondered if I'd get better performance from the radiators if the loop
was replumbed to be 22mm all the way.

Well, 15mm is in the same ballpark as 6 radiators, although we can't tell if
it is officially OK without knowing the radiator outputs. Also, the likely
consequences of slight undersizing is a slight increase in noise, or,
alternatively, a slight reduction in performance depending on the pump
setting. The limiting factor here is the speed of water in the pipes, which
should be kept low to avoid noise.

My house had only a 6kW heating requirement via 8 radiators, which could
have marginally been done entirely in 15mm. However, I have run the main
unzoned backbone in 22mm and had the various zones spurred off in 15mm. The
maximum on a zone is around 3kW, around half the allowed figure.

However, the 15mm would not be able to feed a modern hot water cylinder
itself. Hopefully, any hot water circuit would be entirely 22mm.

Christian.