I've tested the Ia and the boost seems to be working as expected. When triggered, there's a significant increase of rpm and speed picks up. The mixture needs to be leaned out or the engine starts to run rough.
The video is a short demo of boost being fired up on a Spitfire Ia at practically ground level. The radiator was fully open throughout, so you could probably squeeze a few more mph out of it if it was partially closed.
I could only get 250 mph max out of the Spit without boost (which seems pretty poor). +12 Boost adds another 20 - 25 mph or so.
Interestingly enough, I clocked the bf-109 at 273 mph in level flight without WEP at ground level - both planes seem too slow to me. The Spitfire Ia more so than the 109.
The Spitfire IIa is insane on +12 boost. I clocked it at over 330 mph at sea level before the Cylinder Head blew up and oil shot all over the cockpit!
http://www.youtube.com/watch?v=0veG5w33Bko
The boost is tripped at about 0.50.