38kw combi boiler supplying a shower. 22mm or 15mm pipework?

I am refurbishing a 1st floor bathroom and installing a shower to replace a bath and have a 22mm/15mm pipe quandary. There is currently a newish Worcester Greenstar 38CDi Classic combination boiler and my question is should I redo historic mixed diameter pipework. The boiler has 15mm mains-in and DHW out connections. At some point in the past there was a cold water tank in the attic and presumably a hot water tank somewhere. There is a half meter of 15mm pipe coming from the DHW boiler output which becomes 22mm supplying the bathroom bath and sink 3m away. The 15mm mains water pipework comes up from the ground floor into the boiler cupboard and also is connected to an historic 22mm pipe to supply the bathroom. This looks an easy route that the previous installer took. I was considering replacing all the 22mm pipework with 15mm as that seems to be the general internet consensus from what I can gather but I want to make sure I am not going to throttle the supply to my shower as the aim is to have a GREAT shower. Any advice gratefully received.