practically speaking the benefits are being able to dial in fsb frequencies that you normally wouldn't be able to as a result of them needing gtl reference voltages outside of nominal values, and stabilizing high fsb frequencies, usually 450mhz and above. anywhere below this, and all you'll be doing adjusting the gtlref is slightly reducing the vcc/vnb/vtt voltages. not usually worth the effort unless you have a reason for changing them. only area below 450fsb I'd suggest using gtl ref votlages explicitely is around the 425-440mhz fsb range as they are very vital for proper stability.