Robleesaunders
Do I have to control fans with Afterburner?  I want to set my FTW3 at OC setting so the fans use a aggressive curve, yet I want to under volt with afterburner.  Is this possible?

That's two different things.  If you have the BIOS switch set to OC mode, the fans will bottom out at 33% minimum & the curve is more aggressive.  If you want that to remain that way and just run it as it is, all you have to do is not mess with fan control in Afterburner.  Make sure a custom fan curve is NOT enabled & leave the "auto" setting on on fan control, and the BIOS will do that for you without issue.
You can just use the voltage curve aspect of Afterburner & nothing else, that's basically what I'm doing.

How much of an impact does undervolting have on watts pulled and power efficiency?
 
I've always just reduced Power Target myself and tried messing with Afterburner for a bit but couldn't get anything stable - can't remember but maybe it was because I wasn't lowering Core Clock (might have actually had +Core Clock too).

Undervolted, games are running the same and my card caps at 61c so far.  So much better.  

idshadow
How much of an impact does undervolting have on watts pulled and power efficiency?
 
I've always just reduced Power Target myself and tried messing with Afterburner for a bit but couldn't get anything stable - can't remember but maybe it was because I wasn't lowering Core Clock (might have actually had +Core Clock too).

 
 
Auto at 285W (75%) I got 900mv/1800mhz/10900 port royal
Auto at 380W (100%) I got 1050mv/1935mhz/11600 port royal
Auto OC at 425W (112%) I got 1075mv/2115mhz/12600 port royal
 
Manual at 285W (VF curve) I got 862mv/1905mhz/11600 port royal
 
Using the VF curve to manually undervolt seems to be very effective on these cards.  A high end 2080ti will draw the same 285W and score 8900 in port royal, so it seems Ampere is very efficient sub-900mv.