Stock
FSB: 24mm (hollow I believe)
RSB: 11mm (hollow)
Aftermarket
FSB: Ultra-racing 25mm (think it's hollow as well) or. Whiteline 27mm .... so not much of a difference from stock
RSB: UR has 16mm/19mm solid bar, can go upto like 26mm (whiteline) from memory.
So I think what curtis is saying, RSB is the one you get the most benefit from. A FSB is more of a luxury.
And from experience, I got the UR RSB 19mm, and the difference was light and day.
Hope this helps someone.