Sorrry SD, referred to you as SSD.
We are a strictly VFR only Luscombe on a Permit. We have two of the large venturii, one each side just forward of the doors. These feed into a regulator that taps off 2 inches of Hg for the turn co-ordinator. The remainder of the vacuum is fed unregulated to the DI and AI (no electrical backup instruments).
The instruments won't really get up to speed until in level flight so we set them after climbing to cruising height. After that further climbing would reduce the vacuum but I suspect they would continue to function for some time at reduced pressure, certainly I've never noticed any abnormal precession.
Performance will obviously vary on different aircraft and with different instrument fits. I suspect that ours are not positioned in the best place. They're far enough back to possibly have some turbulent airflow off the cowl and also will suffer from blanking if the ball is off-centre. I've seen other installations where they are mounted side by side on the belly where they will get a cleaner airflow.
I understand a major problem with venturi fed instruments was their susceptibility to failure in icing conditions (just when you need them most).
Mike