I bought a set of those cabin vent things a few years back, never installed them. When it comes right down to it, I've never felt that the cabin venting was lacking in any way. Why did I buy them then? I was writing an article about the Ventubes, and just decided to order the cabin vents too. I may actually try them out to see if there is anything about increased cabin venting that is desirable, probably just before redoing the interior in case I don't care for the "improvement"
I've test flown with the rear plastic bulkhead out of the airplane, and can't imagine that there can be any better venting than that, although to be fair the cabin vent system also adds a vent on one of the rear inspection covers, which ought to increase flow through.
