Leave 22mm or replace

I've replaced my old radiators and rejigged where they sit in the rooms (two smaller spread apart rather than one huge). The existing system now has a 4m length of 22mm that feeds two radiators and just doesn't need to be that size. Would the system run better with the pipe reduced to 15mm or better to just leave in place. Currently have floor boards up so now is the time if needed.
Thanks

 

Leave it as is. No benefit whatsoever in reducing sizes of central heating pipework.