How is the cable to the shower run? (whats it supported on/clipped to?/near?) and is it grouped with any others or surrounded by insulation for any length?
8.5kw can go on 6mm depending on how the cable is run (but not on a 32A MCB, it would have to be uprated to 40A assuming circuit is suitable, and of course the circuit should be fully tested afterwards, also for what its worth, its also notifiable to your local building control department)
2- Simply put a 10.5kw shower is 23 and a half percent better than a 8.5kw one
, its that simple, it takes a given amount of energy to raise the temperature of one litre of water by 1 degree celesis (4.184 Kjoules, or one Kcal if you prefer), an 8.5kw shower (assuming its not thermostatic controlled one), will simply put out 8.5Kjoules into the water passing through it every second, the temperature control on the front is simply a flow regulator, so if you want it hotter, what you actually do is reduce the flow, which is why you have to have it set higher in winter as it controls the amount the water is raised in temperature, rather than absolute temperture. QED
As to noticing the difference, there is a bit of difference between the two, obviously, but both will feel poor compared to say a pumped shower off the DHW system, that can deliver upto say 12 litres a minute* at upto whatever temperature the tank is at, until you run out of hot water
If its complete refit of shower room, I'd be tempted to put in a bigger cable and a 10.5kw shower, its a simple shower swap, then I'd put in the biggest shower that the existing cable could support within the bounds of BS7671
At the end of the day, its a downstairs shower room, I expect it only sees use if you want a quick shower while the missus is trying for the record of how long a person can stay in a bathtub, or if the gas boiler breaks down
*or even more if you have a more powerful pump, but than you start to get in wasteful territory....