Faster is always better even on a daily rig.
What also can be mentioned is that the most noticable increase of performance comes from Cpu and NB over clocks, and as RGone said HT would increase performance with multiple VGA and multiple Cpu platforms.
You can do some reading about HT here.
http://www.hypertransport.org/default.cfm?page=FAQs#faq5
The native Ram speed for AMD processors is not actually 1866 but rather 1333 like it has been for Dual channel mode.. Unless you run a slower Ram like depicted above which boots at 533 when CMOS is cleared, your board will post a slower speed.
you can view AMD's site for this kind of info, but 1600 and up is in fact an overclock on the memory controller. The IMC does in fact "support" higher speeds while using Ram that fits both AMD and the motherboard manufacturer's qualified vendor list.... which seems to not be updated often at most mobo sites....
http://support.amd.com/us/kbarticles/Pages/ddr3memoryfrequencyguide.aspx
The least performance you will notice would be from Ram overclocking and tweaking. Stressing the IMC will cause your Prime95 to come up short as you noticed was cured while lowering the reference clock to 210 or lower.
Passing 210 on current timings and divider would require a voltage bump to the Ram. you can be generous, probably safe at 1.75v-1.8v For 220 reference at your current divider. But definitely cool the ram actively.
What is a good benchmark tester ?
Join Seti @ Home or Folding @ Home.... and just use the PC like you normally would with heavy gaming