I've tried several different pump gas street motors both ways and really didn't see a performance gain with using the vacuum advance
I don't track the MPG either or drive them more than 50 miles in a single day usually
I do work on my carbs and distributors to get the mechanical advance and AFR the way I want them also
I like to see between 14 to 18 BTDC at idle on the timing and between 34 and 36 degrees BTDC above 2000 RPM, 12.8 to 13.5 AFR at WOT and between 14.2 to 15.3 AFR at light part throttle cruise with the heat range spark plug that motor wants to stay clean at idle and at WOT