How do I get even water pressure from a bathtub mixer faucet?
Delta makes a line of shower valves (Monitor 17 series) that has separate Temperature and Volume controls in a single hole package.  It approximates a thermostatic valve, which is a much more expensive valve.   

The little arm at 2 o'clock is the temperature setting, and the larger one at 4 o'clock is the volume.



It would be necessary to replace both the valve body and the valve trim to use this solution.