This is an older post now, but I didn't get my CRS until recently and found during the warmest months I was running about 43°C on my CRS210. Room temps were around 80°F or 27°C.
I sent an email or two to tech support and they told me how to check the single SFP+ module that I have installed in slot 2, and it was the same 43°C temp or +1°C higher. Probably not anything for me to worry about then unless it hits 50°C or higher I figured.
I thought about doing this fan mod too (nice mod BTW), although the header for the fan is supposed to be inactive (not connected) and the one that works for this mod is at the supply voltage to the left. So, I'd have to find a small 24V fan or something that fits at a lower voltage and a voltage regulator. I've soldered hundreds of connections through the years and that didn't bug me although my CRS is pretty new yet. I also knew that if I found a fan that small in order to move any air it would be higher RPM=louder.
After a bit of searching around, I ended up finding/using a fan called the AC Infinity MULTIFAN S2. If you look this one up online, it's a blower type fan that has a USB power connector on it. I have my CRS sitting on top of two blocks along the edges so that I can run the cables underneath and back off my desk. Since I'm using this as a desktop unit there is not much airflow under the monitor shelf. Since the S2 fan sits flat, I put it on circuit board standoffs and rubber feet to align it with the back of the CRS. Noise is pretty low compared to my PC running although I can hear it if I listen for it. If there is an air gap where the fan does not touch the case, the noise goes up a lot.
The results after hooking it up tonight look good after waiting at least a half hour; on the lowest fan setting "L" I dropped from 40°C to 30°C; a full 10 degrees. Room temperature here is currently 75°F or 24°C. The SFP+ module is now the same temperature as well, 30°C. I expect to get my S+RJ10 in the next week or two and that's when I decided to try the fan idea.
So, while not quite an internal fan that would fit on a rack, it doesn't require any modification and is fairly quiet if you also use your CRS as a desktop switch. The hardest part will be getting the fan to line up with your back of the case, but if you have a good assortment of standoffs already and some rubber feet you can remove the ones that come with the fan and get close pretty quick.
Keep cool.
