Electric Showers is higher KW better?

Yes it will improve the flow but you will need to checkout the size of your existing power feed as you may need to install a bigger power feed which will have to be routed back to consumer panel so it could be a double price whammy certainly for the 10.5kw. I am not an electrician but I had the same problem which prompted me to install a shower valve and a 3 bar pump to my own hot water system which is an electric heated system. These systems now come with a pressurised tank which doesn't need a pump. I seem to remember an alternative electric shower that has a relatively small dedicated hot water tank which is concealed close to the shower which could help or if you have a combi boiler it has it's own built in pump but you will need to speak to a local plumber . Hope this has given you an alternatives to consider