Almost everything except the Spit Mk.II is slower than it should and that's what Luthier probably referred to when talking about FM fixes (i think he was replying to a thread about this very issue).
With that out of the way and without going into the octane/boost levels debate at all, the root cause of the problem is the fact that most people are not really familiar with how prop pitch works and are getting confused with the two-speed prop.
The trick is simple but a little time consuming, however it should be no big deal for any dedicated Spit drivers until we get a Rotol (constant speed) variant:
start a free flight mission and just fly for half an hour with the sole aim of seeing how much you can get away with without frying the engine.
I'm almost not flying the Spit at all but i have no problem getting 250mph IAS at medium altitudes (which would translate to about 300 mph) with very conservative cruise power settings: no over-boost, just maximum continuous power (the white triangle marker in the instruments red zone), coarse pitch and being careful to trim well and minimize radiator use.
The problem many people have is that they don't know much about how the propellers really work and what that means for ease of maintaining performance. It's no biggie, i didn't know either, there are a load of articles on the web and after some reading my handling with all aircraft improved considerably.
I've seen three excellent articles posted here some time ago, some of it might be too technical (actually the mixture article is mostly about modern civilian prop-aircraft from a point onwards) but it's a worthy read. Just take your time when reading it and don't rush through it, you'll see a lot of mysteries getting explained
Here are the links
manifold pressure:
http://www.avweb.com/news/pelican/182081-1.html
propellers:
http://www.avweb.com/news/pelican/182082-1.html
mixture:
http://www.avweb.com/news/pelican/182084-1.html