So, not an ASUS board, but this appears to be the best source on Ivy Bridge information at the moment.
I've been trying to bin 3770Ks to find one that will do 4.9 GHz or more, but I haven't had much luck. The four 3770Ks I've had have all landed at 4.7 GHz or less. The two Costa Rica CPUs from week 13 of 2013 I've tried both do 4.7 GHz at around 1.36V.
I decided to see what it actually took to manage 4.8 GHz or 4.9 GHz - I pushed these two to 1.515V and 4.9 GHz still failed stress tests almost immediately. 4.8 GHz passes stress tests at just under 1.4V, but still got WHEA errors at 1.515V.
Using a Z77 OC Formula with a garbage power supply. The CPU is running off of one 8-pin CPU power connector - the extra 4-pin is not being used.
Temperatures aren't out of control causing instability - CPU temperatures are staying under 70°C under load. I have no intention of running 1.515V daily, but the fact I can't stabilize an extra 100 MHz with an 11% increase in voltage is a little bit unusual to me. Could something else be going on, or did I run into an absolutely absurd voltage wall?