I don't think that the level of connection chosen is the issue with the amount of hot water that is available. This has been discussed befoe & it would appear that this relates to the size of the hot water tank.
I have been told on the main forum that the 5.75kw connection should be sufficient fora 2 bed apartment unless you have all your appliances running at the same time.
It is apparently quite easy to increase if there is a problem. What you also need to realise is that not only do you have to pay a higher connection fee but the unit rate of the electricity also increases with the higher kilowattage.
Noreen