I have 2 GTX 780s w/ ACX  in SLI. The top card runs hotter, which is to be expected, but it is significantly hotter when gaming. It's running about 76-78 and has hit 80 a couple times even with fan profile set to 100%. This seems to be a somewhat new issue for me. A few months ago it would rarely go over 70.  Would replacing the TIM help? I know eVGA has improved their TIM since the 8800GTX days but is it possible that my goop is drying out?

No changes at all. That's what's so weird. Unless the fans are crappin' out. They look like they're spinning fine but idk?
 

Do you have one Open Slot between your Cards?
If not the Top Card will run Hot.

OK, it sounds like you not sure about temps you should be getting from your Sli ACX cards.
I was under the impression that they were running cooler until now, but it sounds like you not sure.
Just so you know ACX are good coolers but personally I think even with good airflow in your case they are not intended for Sli, one will feed the other with hot air.. period, regardless of their spacing. As a single card, yes ACX cooler is great. Now! that doesn't mean people don't run them in Sli.
Some people run them 2,3 even 4 way Sli, and as long as the temps you seen are in your comfort zone it's up to you.
Blower style will always run slightly cooler as the some if not most of the hot air exits through the back.

My 1 780 classified card hits 80 to 85 playing stwor and firefall at 2650x1440. It's in a corsair 800D with three extra fans blow on it.

Yes, I just dusted it out the other week.  At idle its stays at around 32 C but when gaming, it jumps to those temps. 

bcavnaugh
80C is ok. But users have been warned not to run Furmark
I don't really see good Air Flow in your case!
What is that fan on the back that is blocking your GPU air flow?

 
 

 
 
That is actually an exhaust. The idea is that it pulls air away from the GPU put the back of the case. It worked extremely well with the blower style coolers as it helped pull heat out the back & away from the GPUs. I left it on with the ACX coolers with the Idea that it would help pull cool air into the space between the cards, which it does, just not as well as I'd hoped.
 



 

I am an idiot.
Well it took me this long to figure it out, but today I decided to remove the fan filters on the push side of my Rad. Guess what happened... I have air flow again. After 2 hrs of GTA V, GPU doesn't go over 70 (64 average) & CPU maxes out at 60 (54 average). Before I could feel lots of hot air coming out the top of my case & thought that couldn't be the culprit.
As soon as I started it up without the filters I noticed 2 things.  There was a lot more air coming out the top of my case and thee was a lot less noise from the fans. The air that comes out now doesn't even get that hot due to the better air flow. Now I'm going to go smack my head off the wall & go to work.