After performing the mod, don’t be hasty in attaching everything back. Try the PSU alone first. Plug the PSU to the wall outlet and connect contacts 4 and 6 (or 3 and 4, as in the picture below) in the ATX connector with a twisted paper-clip:
Measure the voltage in the +3.3v circuit with a multimeter and rotate the variable resistor to reduce its resistance. You should have patience if the resistor has many revolutions – you’ve got to do a lot of turning. Anyway, the voltage will start to grow sooner or later. Rotate it to the desired level, and then return to the norm. Install the PSU into the system case, repeat the procedure in a “live” computer using the BIOS data or the multimeter for orientation (after a small additional mod, see below). That’s all, the mod is over.
Your courage alone determines the voltages you set. The limiting voltages of the standard control module in the Antec TrueControl 550 PSU may be considered as safe (this is an off-the-shelf product, which should have undergone strict factory tests). These are about +3.45v, +5.4v and +12.5v.
I heard reports from users who had been running systems with about +3.6v, +5.5v, and +12.9v for months. This is a sensible limit for everyday use, I guess.
As for breaking any records – “sky is the limit!” I know cases when the +3.3v rail was outputting +4.1v, but no one will ever warrant you the safety of your particular hardware at so super-high voltages… Do it at your own risk!