I've been digging into this topic lately and I think it may be an issue on my current build. I did some worst case calculations that I believe have merit, I have made some simple formulas to help calculate proper air filter sizing. To simplify this you can essentially treat the motor as an air pump. So based on its displacement, you can figure out how much air the motor requires. When you add a turbocharger into the mix, things get a little more complicated, but essentially as you build boost, you need more air, pretty simple I believe. You need the increase in air to increase the pressure in the piping. My calculations really simplify things, but they do hold merit. Also another note is that as the RPM increases, the air requirements go up as well.
On my TIII Holset HE341 powered R/T I am running an AEM 21-203DK. I have back calculated this air filter to supply 550CFM. Just to lay out a few more details, I am running a 3" intake from the turbo to the filter location between the IC and the radiator to get cooler air.
Using these worst case scenario calculations I have calculated a required CFM on my car of 1100 CFM, this is base on a RPM of 7000 and a boost level of 35 PSI. So looking at these numbers, it looks like my filter is undersized. I can tell you that my car has no issue hitting 35 PSI, but it does feel like as the RPM's climb the car does not pull as hard, it feel sort of restricted to me.
So my theory is the turbo can supply the required pressure that I am commanding, but I believe that it is spinning faster that it really needs to, to meet this boost level. I think you can treat the air filter as a restrictor and I would not be surprised if there was measurable vacuum in my intake pipe. I also believe that since I am spinning the turbo shaft faster than really required for this boost level that there is more back pressure in the exhaust manifold than necessary.

Does anyone have any real world testing of these theories, did you swap filters and pick up power or remove the filter altogether? Has anyone measured the vacuum of their intake? Has any one measured turbo exhuast drive pressure? I really think this is some low hanging fruit that cold make a measurable difference. I have a fitting tapped into my ported manifold and I plan to measure my exhaust back pressure and my intake vacuum level. I can supply my formulas or spreadsheet if anyone else wants to dig into this. I may be a rare case with the boost levels I am pushing on my car, but this data could be useful on any turbo builds.