We need more details before we can give you a answer, Are you in the UK? *8 master baths complete with showers, Where are the other 4 shower rooms going in? What's the PSI in the water service?
I have two 300ltrs unvented cylinders
Are we talking water heaters or storage tanks? What so the shower rooms consist of?
15 mm dropping down to 22 mm
This doesn't make any sense at all. Please post back with more details. Back to you, Tom