If you have tried to run Devhook to emulate 2.71 at 333mhz and discovered that it freezes, I have found a trick to fix that. But you have to have iR Shell.
Run iR Shell.
Press R+Start repeatedly until the PSP is clocked up to 333mhz (Check the green numbers at the top of the screen).
Run Devhook 0.46 through iR Shell.
Choose "CPU Speed".
Choose 333/166.
Choose "Start".
The PSP will successfully boot 2.71 firmware clocked at 333mhz.
So there you have it! This is great for getting less-choppy Flash movies. Sadly, WiFi is still broken with 333mhz.
I am guessing the reason this works is because Devhook doesn't clock it up to 333 until the point where it usually freezes. But if iR Shell already has the PSP clocked up to 333, the CPU speed isn't affected during the boot process.