Intake cone filter and air pressure

The Pipercross airbox I bough was supplied with a built in air-filter rated up to 200 hp. As don't want to strangle my engine I now have fitted a cone filter with larger filter area.

102mm to 80mm silicone reducing elbow from do88, a plastic tube and three hose clamps from the local hardware store. The filter is left overs from the CSR200 engine.

As I already have a MAP-sensor connected to ECU, I decided to do a road test and see if I could see any pressure drop at the intake. Ideally I would compare the three different filters against each other, both regarding pressure drop and maybe 100-200 km/h acceleration, but I rather not on public roads.

RPM vs air pressure (click to enlarge)

The lowest pressure seen was 0.9918 bar, which equals a drop of 8.2 millibars or 0.82%. In theory that would decrease power with the same amount, or in my case 2.2 hp*. The pressure increase seen on this chart is interesting. I have an hunch of what's happening but I don't have enough knowledge to either explain it or fully understand what the result is. Could it have something to do with air moving but the engine's volumetric efficiency is declining and that causes pressure build up?

I was worried the intake temps would suffer with the filter close to the exhaust manifold, but it appears that they are lower than they used to be with the big sausage type filter and never went above 28°C, with the ambient temperature being around 18°C.

* Source: Four-Stroke Performance Tuning page 26, A. Graham Bell.