Danni, where is your cold water feed to the shower coming from? I've got a 10.5kw shower that is fed off the cold water from my cold from my unvented hot water tank. It used to work like crap, but then I found that if you turn the shower on then turn a tap on for a couple of seconds that is on the same supply (is from the cold after the hot water tank) then the flow to the shower increases and it gives a lovely shower.