I don't really see the point of a rad for a safety net...just set temp thresholds in both BIOS and Software and you have two levels of safety against overheating. Use the software shutdown as the first lower temp since will take a few seconds to shutdown rig since its a "clean" shutdown, then set the BIOS a few degrees after that so if things really go bad and its still heating up while shutdown is running it hits a danger zone and instantly shuts down. That way you are still protected, but won't be sacrificing cooling performance 100% of the time for something that should rarely if ever occur. In years of TEC use I've only had it trigger once due to a dead pump.
Instead of a rad, use a decent insulated res. As long as you don't turn your computer off constantly it works great as a buffer. Down side being that after a cold boot, you will have a few minutes of warmer temps until the rad buffer is cooled to target temp.