All domestic hot water systems in France should be closed mains pressurised be they ballons or combi boilers, both types give balanced hot & cold water to the shower at mains pressure.

There has never been a need for shower pumps unless a P&O builder/renovator has fitted a UK header tank type hot water system.

The pressure and flow from your upstairs or downstairs showers are not related to the position of your ballon whether it be on the ground floor, first floor, loft or cellar, they are both at mains pressure which will be for example nominally 2.0 bar at the foot of the building and the pressure will drop 1 bar for every 10m of elevation.

Your first floor shower will have a pressure 1/4 of a bar less than the ground floor one, unless you have low mains pressure to the house its doubtfull that you would notice the difference in flow, its more likely that the pipework is undersized and restrictive with many elbows etc or simply the mixer or shower head is partially blocked with calcaire.

I still laugh when I see the self proclaimed Engineer Straw Dickbridge huffing and puffing carrying a pressurised ballon d'eau chaude up 4 levels to the grenier of their Chateau when it would have worked just as well if not better in the sous sol where they could have wheeled it straight in from the van.

JohnnyD, water pressure here in France is much higher than in the UK, the reason why you see many of these things, with or without meter. To keep the pressure below 3 Bar. Reading stuff, also available in English.

JohnnyD, every Bricomarché, Leroymerlin etc. has several models. Your amazon example is one of the many models available. Before buying you should know if your water pressure is really (much) higher than the 3 Bar level. If the local water tower is on a high hilltop nearby and you live down in the valley, you can sometimes encounter pressures up to 7 or 8 Bar. When you close a tap and hear a knocking sound you probably have a rather high water pressure. More about water hammer.

Water hammer problems, also helpful: this one, from Flamco. And a French name for it, see this package.

JohnnyD, that's why I already made some PR for Flamco......Your on/off valves will suffer, they are not going to last very long, expensive. Trust your ears and ask your neighbours if they have installed a régulateur de pression. If so, buy one.

Safety first, also additional Flamcos here, this one for our washing machine.

Measuring water pressure, not difficult, not expensive, see these quality Rothenberger tools.