I love my Pi, but it takes a considerable time to boot. If I want to use it as a Media Centre, it's quite important that it powers up quickly.
Are there any steps I can take to improve boot performance?
Certain things can be disabled that improve boot up speed - swap for instance (my Pi takes ages to activate it.)
However, SD card reads are comparatively slow, and the device itself isn't all that fast - I don't think there's a magic bullet to make it boot up that much faster than by default.
I know it's something you've considered already, but personally I would just leave it on. The two main reasons for not doing so appear to be stability (which could be solved by a cron job reboot every so often if it proves to be an issue at all) and SD card wear. While I don't have any figures on how quickly the latter will take place, there's things you can do to mitigate it (disabling swap) and for the price of SD cards, I'd be tempted to just see what happens. If it dies relatively quickly, you can revisit it and you've not thrown away that much. If it lasts for a couple of years or more (which I highly suspect may well be the case, though I have no benchmarks for that) I think £10 every few years is a rather low maintenance cost in the grand scheme of things.
As mentioned already the main thing that kills SD cards is lots of writes, and I can't think of anything that would really be doing that (or should be doing that) when the Pi is sitting idle.
If you are willing to switch to a whole new init system, I've found that using systemd allows the system to boot up considerably quicker, although I haven't done any benchmarks to give any real numbers.
A few suggestions: Strip SystemV at /etc/rc.sysinit
or Use systemd
Parallel load the programs
Compile the kernel with hardfloat
E.g /usr /bin /sbin .. etc..
External SSD and HDD for the root partition