You are correct, the vacuum pot(s) only retards the timing during off-boost events.
Contrary to popular belief, the '78/9 distributor does NOT retard timing under boost. You can blow on that bugger 'till your eyes pop out and it won't retard the timing.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8