I performed a super battery life mod because I need it for tuning ECU's on cars, sometimes it takes a day and it is comfortable not ot be "wired".
I used R51 Intel mobo, 14.1 XGA LCD from T42, Pentium M 750 (1,86 GHz running on 400 MHz FSB = 1,4 GHz and undervolted to the max, works perfectly on R51 mobo), no modem/BT, disabled all ports except COM1 (soldered wires directly to MAX3243C and fitted COM port to optical drive travel bezel) and touchpad. HDD is Hitachi 5400rpm drive.
It really does have significant impact as with Panasonic 9cell (BM shows 11% wear) it holds laptop in work for around 7 hours. Note that the user interface program that I use (VemsTune) is medium in resource requirements (around 100 MB memory, CPU hovers around 5-8%).
I am really glad how it worked out and this way you can have many things on T4x series: looong battery life, rigid laptop and durability. Now I have to find a palmrest that does not have touchpad
Not sure how well T43 board can be made to save power but SATA mod definitely helps.
X32s:PM 758 LV CPU mod,2GB,64GB microSATA SSD,COM mod,IPS