New ODE V2 Level 4 (Kinda)

BACKGROUND
Budget:
$2k
Expectations:
Online Gaming - GW2
So I got me a Black Friday Baby! ODE V2 Level 4 (with 1 GTX 670, not 2). Love it, Love it!

KUDOS to DS!
DS really came through with superior customer service as the Vortex 240 cooling died within a week. down for a coupla days until the warranty unit arrived and ez install back up and running.

DISCUSSION
I am looking to upgrade the fans because the noise level is crazy loud. After researching all options, i am so ready to pull the trigger on a set of new 120mm fans made by Noctua (NF-P12-1300 or more likely NF-F12 PWM). The fans are plugged in to the ASUS z77 Sabertooth as follows from DS:
1. 200mm stock Corsair fan is plugged into the chassis fan controller
2. One of the top 120mm Vortex fans is plugged into the CHA_FAN2 header.
3***. The other top 120mm Vortex fan is plugged into the CPU_Opt header
4. Rear chassis fan 120mm? (i think?) is plugged into the CHA_FAN3 header. Interestly, ASUS recommends that this fan be plugged into CHA_FAN1 or CHA_FAN2.
5. The DS Vortex Cooler (radiator) is plugged into CPU_FAN, which makes sense.

the question is what is this CPU_OPT header all about? it is not even supported like all the other fan attachments in the BIOS.

Also, from what i have read, nobody should ever change anything regarding the Vortex radiator and CPU_FAN... leave that one alone as set by DS pros.

When i get the Noctua fans i plan on plugging the top two 120mm into CHA_FAN2 and CHA_FAN3, and the new rear Noctua into CHA_FAN1 (as ASUS recommends) and thus leaving both CPU_OPT and CHA_FAN4 empty. thoughts anyone? thank you in advance...

kimosabe