The Kinetico 2020c High Flow Softener has a service flow rate of 33 lpm
and
a peak flow rate of 51 lpm. Providing that I use a suitably large pipe
diameter, is this really going to appreciably slow down my 30 lpm supply?

You'll probably be alright. 2.8 bar is plenty strong enough to push through
a restriction. If your flow rate relied on wide bore pipes under low
pressure, the effect would be much greater than your high pressure system.

In any case, I suspect your supply might be capable of more than 25lpm. The
water company is only interested in there being more than 7lpm or 9lpm or
whatever their standards are. If they get 25lpm out of a thin bore mains
pressure only kitchen tap, they're going to say "that's fine" and not
investigate further.

Christian.