I'd agree the pan-evac effect is weak at lower RPM based on my own experience, but an exhaust past the collectors could be a factor.
Briefly, installed a fresh engine that included a low-tension setup, otherwise very similar to the previous unit (cam, heads, intake, + 1.0 CR).
Has a huge, deep pan, mesh tray, 10-W30 oil well away from crank.
Had previously added 3.5" X-pipe & Race Bullets, ending @ rear axle. Had pan-evac with proper 1-way valves & angled suction @ collectors.
New motor would smoke a bit through the exhaust in the lanes & such, 1500-2000-ish RPM range. Would clear up downtrack. Shift points 7500.
Changed to an electric GM vac pump & puke tank, no smoke. Checked vacuum via fitting in opposite VC, 5" vacuum.