If you want to do a fair comparison to see natural gains of one of these style manifolds, make sure it's done on the same day, same weather, and same dyno. Also make sure ignition advance is reduced enough to eliminate ignition retard from knock during both tests but keep the same ignition advance tune for both tests. Air intake temps, boost and fuel curves need to be mirror images. This leaves only the volumetric efficiency and mass flow as the true measured variable. This can be measured and compared on MAF cars. If it truly helps improve the engines natural ability to flow more, this will show it.
On a small turbo setup like a K04 or smaller, the true culprit to reducing your engines ability to flow is the turbos turbine housing. It starts creating so much back pressure at high boost that it begins to block it's own flow. So when the exabust valves open and try to expel all of the old burnt exhaust gases, it can't. This leaves some if the old inert gases in the cylinders while the new fresh air is being forced in. This is where the volumetric efficiency of the engine takes a nose dive.
For those who don't know, volumetric efficiency is the engines ability to flow it's natural displacement of volume. The engine has 2.0 liters of volume but if you can only fill the cylinders with 1.8 liters of fresh air/fuel during every cycle it's only using 90% of it's volume. So we would say the VE is only 90% at that cycle. The VE will vary at different RPM ranges on each setup. Yes, the right design can see VE greater than 100%
My best example of the turbos back pressure starting to ruin the engines VE,lol:
