electric shower selection - low pressure

I have a variable and low mains pressure in my flat. I have a combi boiler and the cold tank is at the same height as the shower, quite small and fills up very slowly due to the poor pressure. It is too risky for me to pump from that (instead of using the mains) as it is very likely to run dry.
I have a Triton T80si 8.5kw which requires 1.0 bar min pressure. I am about to replace this with a Redring or Aqualisa shower which is listed as having a lower minimum pressure (0.7 bar).
My question is, in my circumstances would I be better going for a 7.5Kw instead of a 8.5kw? or will that not make any difference?
thanks a lot
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