I agree with DrTeeth, the only way to find out (for you) is to try it.
I have to say though that I have upgraded my RT-N66U from each firmware within days of it being released (RMerlin's and Asus') and as long as I didn't take any shortcuts (like; not doing a full reset and full, manual, reconfiguration) each update has been good to great.
The sdk-5 firmwares are essentially obsolete now as RMerlin isn't updating that branch. To give yourself the best chance of having no issues with the new firmware:
1) Unplug all USB devices from your router.
2) Unplug all Ethernet cables except the one you're using to connect the computer that will configure it.
3) Restart the router (I would suggest by pulling the power plug, not a reboot via the web gui).
4) Upgrade the firmware.
5) Reset to defaults and do another hard restart (pull the power plug).
6) Setup the router as basic as possible (we only need to access the internet right now). I would also be using new ssid's or removing/forgetting network connections from all devices and then re-associating each device (easier to set a new ssid instead, imo).
7) I suggest another reboot here (has solved many problems for me).
8) When the router has been left to fully reboot (I wait a good 5 minutes here), continue manually setting up the router for your needs. Changing one major feature at a time and testing for a while is a good way to go.
9) When it is fully setup as you need; pull the power once more (on both the router and the modem), wait a good 10 to 15 minutes, power on the router first, (waiting 2 minutes), power on the modem (waiting an additional 5 minutes).
(What powering off the modem does is resets your connection with your ISP and hopefully gets rid of any glitches that may be introduced by not doing so).
You may also want to try the suggests in the thread linked below.
http://forums.smallnetbuilder.com/showthread.php?p=104073#post104073
(I have to make a single post on all of the 'best of the best' settings one day).
Now, if your experience is anything like mine: your network will be rock solid again until you mess with it again.
I am on the 374.40 firmware since it was released (~9 days...) with no issues whatsoever (at least; performance wise).