Just had a reply from a pilot who often flies old warbirds (including Spitfires, P40's, Wirraways etc) as well as many other types of aircraft. He stated the boost gauge goes down when the revs drop on supercharged engines, regardless of whether it's due to increase load on the engine.
It seems CoD is wrong and A2A is correct.

Thanks very much for that info.

I've also done a bit of asking (could only find a Mustang pilot though, and it seems the P51 has some sort of automatic boost control, so it pretty much stays where you set it, within limits - it was a short convo, and I didn't want to press too hard?), and this confirms what I've suspected and the info I've seen.