I replaced cartridge and cleaned what calcium deposits I could. Still have half the pressure I should have

When you turned the water back on, some debris in your supply pipes could have made its way to the back of your cartridge reducing your pressure. Or, your shut-off valves are not fully open. You can try to flush out the valve by opening up the shut-off valves a little and allow water to flow, and then try rinsing out the cartridge and reassemble. Hope this helps.

If the pressure is OK at the tub spout then you may simply have a clogged shower head. In this case, remove the shower head and run the water without the head in place. Clear the shower head of any sediment/gunk and replace.

If that fails to work, then follow Scott's advice above... ;)

Mark