The Old Man Emu (OME) torsions bars are an optional add-on to their lift package. The stock Toyota torsion bars may sag over time; however, they can be adjusted to compensate for the sag. The benefits of purchasing the OME torsion bars are that they are new (no sag) and are a larger diameter than the stock bars, yielding more weight carrying capability and resistance to bottoming out (good if you have a winch and/or heavy bumper). There is no reason you will have to replace your stock torsion bars unless they feel too soft or are damaged. Front lift can also be achieved with the addition of ball joint spacers.
As for size difference:
Old Man Emu = 24mm diameter
Original Toyota bar = 22mm diameter
Since the torsion bar's resistance to twisting (the measure of its spring rate) is based on diameter, length and material used you would need to use the equation the engineers do (known as diameter to the fourth) in order to compare the two bars to each other.
Take the Old Man Emu bars at 24mm, 24 to the 4th = 331,776
Do the same with the stock 22mm bars = 234,256
Divide the new bar by the old bar and you will see that it is approximately 41.6% stiffer than the original Toyota bar. That is a pretty significant difference, but for many people is more than within the realm of comfortable and usable.
