It really depends on how the timing is set. My car runs best at idle with 18* of timing, so, even with the vacuum advance adjusted to its absolute minimum I was getting over 50* of advance all in, which is way too much and caused detonation. With the initial at 18*, the mechanical set up for 36* of total advance and the vacuum advance unhooked my car runs MUCH better than it ever did with the vacuum advance hooked up. 36* is plenty of advance. So, the idea that not using vacuum advance means you're not getting the most out of your engine isn't accurate at all. You just need to make sure that the distributor is set up to give you the advance that you need.