[robg pre-adds... Before you contemplate trying this hint, please read the comment with the tech note explaining why you should not do this. I'm leaving the hint in place as a warning in case someone finds the info elsewhere and wants to see what macosxhints might have to say on the matter.]
On my new PB15 (7457-based) I discovered that the Branch Target Instruction Cache is disabled. It was enabled on my prior Ti-Book (7450-based). I don't know why it's not enabled, perhaps the OS (10.2.8) still does not recognize the newer chip fully, and is playing it safe. Turning it back on does not appear to cause system instability, and increases performance slightly (3% on integer-intensive benchmarks).
The easiest way to turn the bit back on is to download the latest CHUD performance tools (3.0.1) from Apple, run the Reggie SE app, click the HID0 tab, then look for bit #26, BTIC. I don't know how to keep it on between reboots, though.
Be careful with Reggie SE. Random clicks in that tool are a sure way to lock your system up.
[robg adds: It took a bit of digging on Apple's site, but I finally found (on the Performance page of Apple's Developer site) a download link that will mount an FTP server in the Finder. The 3.0 version is a 21mb download. Anything that may not work on your machine after tweaking it in this manner is, of course, your own responsibility.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031014100802290