Since dyno runs would be at WOT, and 0 inches of vacuum, I would not think the vacuum advance would have any impact on peak HP/TQ numbers.
It could effect economy or part throttle/cruise performance.
Was there any definition of what they meant by "doesn't play well"? Is it taking timing back out when it sees the vacuum advance try to run it up for economy? Does this lead to surging at cruise/part throttle driving?
DynoDave Walter P. Chrysler Club - Great Lakes Region Member # 12304 1970 Plymouth Duster 1972 Dodge Charger Rallye https://wichargerguy.proboards.com/ 1977 Chrysler Cordoba