I'd say exactly what Nate said, also JJ from Asus has really good videos on over clocking auto(which most users I've seen run into issues with
Manual/offset which most prefer I believe and the newer one of them the adaptive

Although JJ warns if ur using adaptive don't use synthetic tests like prime95 as it can add a multiplayer to our voltage and make it jump to awful and way way to high of voltages

Although the haswells seem to have nothing in common from 1 to another, some over clock some don't, some that over clock then can't over clockram, some can, some can get oover clocked and still under load sit around 50-60c while some stock right out ta the box with a decent cooler and no over clock sit at the same temp lol, pretty much the haswell lottery lol

I did see an interesting thread on a different forum where someone over clocked a 4770k with the stock cooler to 4.4 at 1.23v and it sat at 66c during stress test, eventually it crashed, most believed to low if voltage at some point but still very good cpu he got compared to the countless threads I've seen of ppl running them stock with a h100 and still having higher idle temps then his stress temps, he was also able to run the cpu at 4.0 and 4.2 while under 1.0v, I believe he had 4.2 at .998v and 4.0 at .789v, again not stable, crazed during stress test due to to low voltage but again that guy seemed to hit the haswell jackpot lol

He also says on haswells at least on everyone he's used 1.3v is usually the very max you could have (not with stock cooler tho) with good aftermarket air or all in one water cooled, says u can usually go a little higher with custom water loops or extreme cooling but that voltage as he said will deg race the cpu much much faster

As for the video card again as Nate said, I usually like trolling thru the forums of said card type to see what other have to get a basic limit of what no one has past, although with 3 of those 290's in crossfire with 4.2-4.4 even with a small over clock on the GPU I'd think I'd probly be getting one of the best if not thee best video performance out of all cards out to date, well maybe the 290x's in the same set up might be the best but that if you wanna drop like $2000 on just the gpu's lol

You could try it. Set the gpu to stock settings if you still get the flicker.

Seems like I got it good at 63 mV. The CPU however is harder. I got it stable at 4.5 Ghz @ 1.25V, but I get a lower score on physics in Fire Strike. Any idea why?

I'm not sure why you would get a lower Physx score, try running fire strike again.

I just did again. 8651 was my Physics score, then with 4.5 Ghz it was 7993 and 8093.

Is your gpu still at the same speed? If so, your score should have gone up a little not down.

My GPU is the same and gets the same Graphics score every time(11 700-11 900). I haven't touched the GPU since I got it stable.