Unless I've got this wrong, Dave, several things don't add up here.
I guess you have an unvented cylinder now, which will deliver hot water at mains pressure. With this, the position of the shower head relative to the cylinder is not relevant. It's at mains pressure, so the water will happily run "uphill".
The Croma needs 1 bar minimum, so 2.25 is plenty. I imagine it's simply the wide-bodied design of the head that gives a completely different shower from the older type.
I'm also surprised that the first floor shower runs at all, especially at the right temperature. A gravity based shower is low pressure... mains fed is much higher. They are completely different things.