The valve in the wall could have been plumbed backwards, causing the hot/cold reversal when the cartridge is installed correctly. When that happens the only choice is the reverse the cartridge so that it mixes correctly.
As far as the pressure, water pressure is should stay constant through a typical valve such as this. What you could be dealing with is a quantity/volume of water issue which can be caused by the cartridge. Don't have much more to offer than that on this part of it.