Usually timings and voltage.
Generic is usually slow timings 10-10-10-30 (potentially slower EG: CAS 11)
Performance is usually 8-8-8-24 (or faster EG: CAS 7)
Then there's voltage, JEDEC officially specify 1.5v (or 1.35v for LV). Usually generic brands will typically stick with the stock voltage, but some shadier brands will buy more marginal parts and up the voltage to 1.65v so that they survive the 1 year warranty... Performance memory sometimes go up to 1.65v too but that's more for aggressive timing stability.
Lastly, validation, it's more LIKELY that performance RAM is better tested and more likely to be more stable... but this last point is arguable, because generic RAM nowadays is quite stable...
General Advice:
Running memory at 1.5v is the most compatible. Until custom settings are put in, a board will supply 1.5v to Memory on first boot, If your memory sticks don't have timings defined for 1.5v, the board may fail to boot (rare occurrence nowadays, more common with the 2007-2009 timeframe).
performance memory would be CAS 7 at 1600MHz, but these kits are just overclocked CAS 8 kits because to get CAS 7 they typically ask for 1.65v or 1.7v
Good memory today should be able to do CAS 8 at 1600MHz with 1.5v.
Decent Memory will do CAS 9 at 1600MHz with 1.5v. If you up the voltage to 1.65v, you can probably get these kits to do CAS 8 (and some manufacturers sell kits that do that).
Low end memory will do CAS 10 at 1600MHz with 1.5v. Again, if you up to voltage to 1.65v you can probably get these kits to do CAS 9. (Your RAMAXEL sticks probably fall into this category, assuming it's not CAS 11)
Don't look any lower...
How big is the impact of CAS 8 vs 10? not much... maybe 5-10%?
edit: the CAS rating and grade only correspond to DDR3 1600, they don't hold true for 1333 or 1866. Usually things goes up or down a step respectively. For example, for DDR3 1333 CAS 7 at 1.5v would be considered high end, and CAS 9 or 10 would be considered high end for DDR3 1866
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