Pipe sizing - fixed shower head

[Monsoon]

Im just plumbing in my new thermostatic shower the hot and cold feeds enter the thermostatic shower in 15mm.

I'm not sure if I should pipe the output to the showerhead in 15 or 22mm ?

Boiler = combi

 

[Lee-King]

If the inlets are in 15mm, you won't gain anything by doing the outlet in 22mm. 15mm outlet to the shower head will be fine.

 

[Monsoon]

Cheers - I was all set to plumb it in 15mm and then thought with 2 X 15mm feeds into a 15mm outlet I may create a bottleneck.

Is the combi(11l/min) the real bottleneck in this situation?

EDIT: Just to add, Ive measured my mains water pressure at a good 3bar

 

[Nige F]

long as the shower mixer is suitable for a combi , all should be OK