I converted one of my Mark II to separate bias. Perhaps I have well-matched tubes, but I noted no difference in the sound with the ones that were in there.
Note that what you describe alters DC balance only. There could be another, more complex adjustment to adjust AC drive balance between the two tubes. Besides more circuit complexity, it also would demand a more rigorous way of measuring the adjustments for proper balance than just the simple "voltage drop across a resistor" that is used to measure the idle (bias) current through the tube.
It is specifically with tubes that are sufficiently unbalanced to require the dual biasing system that the requirement to adjust AC drive balance would be the greatest. If it's needed but can't be provided, then the use of unbalanced tubes is still suboptimal, even if we have dual bias adjustment.
Note also that when you separate the cathode resistor in a Dynaco amplifier into separate ones, as you must in order to measure the idle current in the two output tubes individually, you are also defeating a feedback mechanism that David Hafler used to reduce distortion in the output stage. So you have to weigh the advantages of using dual bias to compensate for unmatched tubes, and the inability to adjust AC drive balance that those same tubes would likely require, against the increase in distortion from separating the cathode resistors.
All in all, I am thinking that the next time I have to dig into that Mark II for other than routine adjustments, the original bias scheme and single cathode resistor will be going back in. Until then, in the words of Dynaco's tech writer Bob Tucker one time when I called him, it's "let sleeping amplifiers lie."