Yes on two grounds - the combi may well have an internal PRV on its DHW outlet. Failing that it may well require one on its mains cold inlet if the water pressure is above a certain limit - and 9 bar is probably over the limit.
It is good to have hot and cold at point of use similar to each other in pressure.
Secondly once you go above 3 bar or so you will have adequate performance out of most taps and showers, any any more is just more water hammer, flow noise, and wear and tear on taps etc.
When I installed my unvented cylinder, I did away with all the tanks etc. That meant that the cold water pressure at the taps and cisterns upstairs went from "feeble" (about 4' of head), to 6+ bar. When you flushed one of the loos it sounded like a jet taking off on full afterburner! (the worst culprit was one with a traditional ball valve on a brass rod)
So when I did the bathrooms, one of my goals was to tame everything down and make it quiet as well as work well and be easy to service.
So adding the PRV for the cold (the one on the unvented cylinder was not really in the right place to make it easy to derive the cold feed for upstairs from there), was a part of that solution. The other bit was trimming out all of the dead legs of pipe, any unwanted right angle fittings, and using full bore service valves everywhere. It made a
*massive* difference. You can flush and the cistern refills very quickly, but almost silently.