Water flow rate

We have had a Grohe shower fitted and although we have a 1.3 bar booster pump we only get about 6 l/m out of it. To get our target of 15l/min and assuming that doubling the pressure increases the flow rate by 1.5 we would need a crazy powerful pump. So my question is. Do some brands (types) of shower get a better flow rate for the same pressure. If so which ones? Thanks in advance for any help.