I did see some people mention that the process temp can typically go higher than the tcase temp safely.
The 2500K spec'ed Tcase temp is 72.5C while the 2100T's is 65C. I'm not sure this difference is meaningful but it's not large (why would it be?). And crazy overclockers make their 2500K go 10C higher or even more. And why not? CPUs can typically handle 90C or more without issues.
Your CPU will most likely not get that hot anyway. Isn't it supposed to throttle itself automatically as it gets hot?
Did some more fan tuning today, upping the fan speeds while making sure I could not hear them through the cabinet. After MKV playback I'm getting the following temps:
These temps are OK but your methodolgy isn't. CPU temperature wouldn't be my main concern with your setup. You have poor ventilation need to make sure your other components will not become too hot. They're not all designed to take CPU-like temperatures. More efficient CPU cooling would also help some motherboard components but will obviously be of limited use if the air in your case get really hot.
If you want to test your cooling system, you need to put the maximum load on your system, not watch videos. You do this by using programs which stress your CPU and GPU like prime95 and furmark at the same time. You may dispense with GPU-stressing if you don't need hardware-accelerated 3D and can reliably disable the functionality.
If your system get too hot under load and you'd rather not use more ventilation, you could configure it so that it won't go so fast (underclocking) because Sandy Bridge doesn't use much power if you don't push it and I suspect you wasted your money buying a more powerful CPU than you need.
It's interesting to know what temperatures you get at idle and under typical load but you also need to consider the situation where abnormal load might but put on the system due to a buggy program, malware or operator error. This is where underclocking comes in handy: it acts as a safety of sorts. It's best done is the BIOS but you can also do it in software if your BIOS is not clever enough to manage the CPU as you'd like in your somewhat unusual situation. Typically, emdebbed software only needs to keep the CPU from overheating but it sounds like your whole system might overheat due to lack of ventilation.