Even though I know tightening the timings won't make hardly any difference at all, I still did it, I also have 1333Mhz RAM, mine is rated at 8-8-8-24 1.65V
I've managed to run it at 1230Mhz at 6-8-6-18 1.68V, However this is only possible due to the fact I upped the DRAM voltage, leaving the DRAM voltage at 1.65V results in instability at those timings.
You can adjust each number individually, so for example you don't have to go from 7-7-7-21 to 6-6-6-18.
Its up to you whether or not you overvolt your RAM to achieve tighter timings, if you do, make sure you keep your QPI/VTT voltage no lower than 0.5V from your DRAM voltage, you could cause damage to your CPU if you do not follow this rule.
My QPI Voltage is 1.355V and my DRAM is 1.68V so I'm well within safe limits.
If I were you I'd try 6-7-7-20 first, run an hour of Intel burn test or blend mode on Prime95, if it all goes well try 6-6-7-20 and test again.... When it becomes unstable you have to decide whether to up the DRAM voltage to regain stability or to up the timings slightly.