I tried configuring the 1454 value using a couple of techniques mentioned on macosxhints, but they didn't seem to have much effect, at least not on the downstream packets. A little network tracing showed that Mac OS X PPPoE was still negotiating an MTU of 1492 with my ISP. So I did a little digging around and came up with the following solution...
The PPP configuration is stored in the following property list file:
/Library/Preferences/SystemConfiguration/preferences.plist
Open this with your preferred text file editor. Note that you need root access for this -- see appropriate macosxhints on using sudo. Also make sure you save a copy first, and use a genuine text editor like vi or pico, not a word processor.
Search down for the PPP section -- it's introduced by this line:
<Key>PPP</Key>
Immediately following this, you will see an indented list of PPP properties. You need to insert two new parameters as follows (insert the appropriate value if you want something other than 1454):
<key>LCPMRU</key>
<integer>1454</integer>
<key>LCPMTU</key>
<integer>1454</integer>
I kept mine in alphabetical order, so after the edit my file reads:
.....
<Key>LCPEchoInterval</Key>
<integer>10</integer>
<key>LCPMRU</key>
<integer>1454</integer>
<key>LCPMTU</key>
<integer>1454</integer>
<key>Logfile</key>
<string>/var/log/ppp.log</string>
....
Yours may differ, of course, depending on what properties are listed.
Save the file, and then restart your Mac (there's probably a neater way of causing the system to re-read the file, but I don't know what it is...) and connect to the internet. In my case, I consistently measured an increase in download speed of around 7-8KBytes/sec. Hardly worth writing home about, but it's nice to know that none of the ADSL modem speed is being wasted.
This tip works on 10.4 at least, but I haven't tried it on earlier versions.

