Mark
A friend in Calgary has an 05 FC 450 for practice, he rides expert offroad ( ISDE silver medalist ) and is int/exp motocrosser. Once he set the bike up, oil height in the forks, clickers, bar position, etc. he became very comfortable with the bike, favouring it over his traded 2006 CRF 450.
He let other riders take the bike for a moto to get their impression of how it works. Most, at least those that were honest, claimed they liked the bike better than the Japanese bikes that were more commonly known as 'the best'. Although the FC was down on power, what power was there was all usable, and favourable to riding an entire moto at race pace. The bike is not tiring, was controlable in the corner and basically was a huge surprise to all who rode it.
So power may be all that everyone talks about, but, getting the power to the ground with out tiring you out is a topic most don't understand or won't allow themselves to understand. The new 59hp 450 KTM sounds like a show stopper, but WHO can honestly say that they will go faster, for a whole moto on that, compared to their old SOHC 450.
At the same time, easy power is available from proper jetting, the slip on pipe will give you a stronger mid to top, the full system will give you more everywhere and a 4 hp gain, and products like the powernow may help throttle response. In reality, tuning is your best bet, with carefull consideration to where you want inprovement and what will get you what you want.
Now considering you are already riding a 550, a stk 450 is just not likely going to work. I'm sure there is that option out there for you mod a 450 to your liking.
fry