In the 70s after ported vac adv had been in use for a few years & emissions got tighter- & engines ran hotter- manufacturers incorporated a temp switch into the cooling so that once temps reached a critical temp, ported VA was switched to manifold VA to cool the engine. I doubt the manufacturers would have gone to this expense if this didn't work....
This is the description in my Motors Manual 1973-78 models, for GM cars. Others are similar. " ...the thermo vacuum switch is added to the system on some engines. The switch senses engine coolant temp & if temp reaches 220*, the switch valve moves to allow manifold vacuum to reach the dist & advance the timing to & allow the engine to run cooler."
The Crane adj vac adv unit will work down to as low as 5~6" of vacuum which will cover most peoples needs.
You cant compare a 70`s smog engine with a engine built for performance. Those 70`s smog engines didnt have enough initial timing to start with some had redicilous low initial like 2-4 deg and lazy curves so total wasnt all in before 4500rpm.
Most with a performance engine run 16-18 initial some like me even more Im running 25 initial. Some even uses looked out distributors.
There are GM cans that start as low as 2 hg” of vacuum.
I tried both them and the crane and none of them where fully activated at 8hg” that I have in D wich resulted in unsteady timing. You need a can that is fully activated atleast 2hg” lower than the hg” you have in gear on a car with aut transmission. Both a couple of GM and crane cans worked at idle in P since I had 10hg” in P put it in D and 150 rpm drop and 2hg” drop was enough to not get the vacuum can to work with manifold vacuum with my combination.
Same thing can happen if you use To light springs so your mechanical curve has started at idle in P but not at idle in D because of the rpm drop between P and D.
My car runs same temp with 16 deg initial as with 25 at idle in line up traffic no matter of time and outside temps of 90+ deg, used 16 deg initial with my Hei and now 25 deg initial with my msd pro billet both had a total of 35 deg all in by 2500. Still ran 175-180 deg in the head or 155-160 in the intake with a 160 deg thermostat with both settings.
So my experience is that if your cooling system is marginal from the begining it may have effect if you use ported vs manifold but if it has some extra cabability no difference to write home about.