I think I've read everything that's been written about putting AMD K6-2 and K6-3[+] processors in iOpeners, but I want to make sure I've got it:
New iO's (v4, v5) have Split-Plane enabled, and if there's a mP6 they're already set to Vcore=2.0v.
Older iO's (v1, v2, not sure of v3) require a mod to enable Split-Plane.
Max ICCcore for a K6-3+ is 8.5A, compared to 2.0A for an mP6. The stock Vcore supply is set to limit somewhere around 6A, hence the mods to the sense resistor.
The current limit exists to keep Q16 from setting the board on fire. If you can cool Q16, either with a heat sink or by adding dropping resistors between it and the 3.5v supply, the next weak link is the iOpener power brick.
This is all established art, amounting to the the Turbo3 mod, with credit due to others along the way. So, going forward:
Where is the limit to what the 3.5v supply can provide?
I have some 2.4A@19V supplies around (IBM WorkPad) and I think that there are other ThinkPad supplies that are even beefier. For instance, the Lind Electronics auto/air adapter for the older ThinkPads can source 3A at 19V.