670 SLI here, some games have been hit and miss on SLI support or the SLI support is there but the performance isn't quite up to scratch.
However there is an increasing amount of games that use SLI and the last few I've played have been excellent on GPU usage and general performance!
I'm one of the SLI users that will tinker with Nvidia inspector to get a game up to my standards via use of another games SLI profile or messing with extreme AA/DSR settings.
As for microstutter can't say I've really experienced it, BF4 when it first came out maybe but now with BF4 2.0 it's smooth as butter. What held me back originally and produced some stutter (Not micro-stutter associated with SLI) was the fact I was on an i5 3570k, since moving to an i7 2700k it's a whole new experience and everything is just smooth (min, avg and max fps all up also).
There have been some bum nvidia drivers that I had to revert from because they either A. broke the SLI profile of the main game I was playing at the time
B. Reduced SLI performance in general
C. introduced flickering in SLI on some games
but the recent newest drivers have been brilliant and are in my opinion the best yet for a long time, everything I currently play (BF4, sniper elite 3, elite dangerous, star citizen, payday2 to name a few) is running better than ever and with the addition of DSR in the driver it's been a god send not having to change my resolution constantly to down-sample although I had to play around to get DSR as sharp as native down-sampling.
Heat generation has been my only gripe even with the excellent DCUII coolers on my 670's the top card can still get above 70c in the most demanding games, not exactly a problem as it tops out at 74c but I had to disable the Kepler throttle

on the plus side the new 900 series runs cooler than Kepler and depending what coolers you have on your cards it's probably a non issue.
For the games that are single card only you are going to be no better or worse except PhysX games that have a rubbish SLI profile, you have the ability to set the 2nd card as a PhysX card which actually nets some decent gains even on an i7.
Your PSU is more than capable (Overkill

) so I'd say go for it and give it a try