DIYbanter - 15mm or 22mm shower plumbing

Hi guys. I'm just about to install a shower pump, and being both lazy
and tight, I devised the most economical way of doing it in terms of
time and money. I suspect that the fallout from this will be poor shower
performance, but what the hell, the bits are ordered now. Anyway, here's
the situation: cold supply from the tank in the loft to the (single)
bathroom is run in 15mm pipe. The hot supply from the h/w tank to the
bathroom is 22mm, but unlike the cold, this is not a dedicated supply as
it feeds the kitchen also. I considered whether to buy a pump with 22mm
inlets/outlets or whether to go for a 15mm fitting version. I decided
that there would be less plumbing to do if I went for 15mm, since the
existing cold could simply be re-routed through the pump and a new
(dedicated to shower) hot drawn from the tank in 15mm pipe via a Surrey
flange. This set up means that the only new pipework I would have to
lay is a new hot from pump to shower (4m) and a little bit of
re-arranging in the airing cupboard. I now have on order a Techflow TP60
1.8bar pump.
Looking at the thermostatic shower valve that I bought, I noticed that
the connections are 22mm. I am now wondering if there is any point in
stepping up to 22mm pipe immediately after the pump outlets, or whether
I should run in 15mm until just prior to the shower valve? Will the
flow be significantly enhanced by 22mm pipe even though the upstream
pipework is 15mm? I guess that the drag of water against pipe wall per
unit volume of water flowing is less in 22mm than 15mm, so it should
make some difference, but would it be significant enough to be worth my
while (the larger bore pipe is less convenient for me to work with in
the relatively confined space I have available to run it).

Thanks in advance, Jim.