I can tell you this, every single device I have ever owned with the exception of my current lg phone got an overclock at some point and kept it. Every device I've owned save a few I sold or got stolen and don't know about still run today. Damage and overclocks have a lot to do with if you exceed heat ratios of the parts. A factor in longevity concerns is also just how long you plan to keep a device as well, most don't exceed 2 years.
Another would be the longevity speculations of the hardware, for instance, your battery will almost invariably lose it's functional life within 2 years under moderate to heavy use. The capacitive screen on average may receive 225 million clicks before borking. Since all of this technology is relatively new time frames can get sticky but I would think it safe to expect 1 year from a heavily used tablet And hope for 3-4 years with a battery replacement.
Therefore when calculating being on the high end of the heat ratio for a SOC with a longevity expectation of 9 years at moderate heat thereby possibly reducing life by a few months would not play highly into my calculated risks. However you must make sure to stay at reasonable temps for your SOC.
HP Slatebook 10 X2 Stock
Nexus 7 2013 - CM 10.2 - 1.9GHz OC - CPU 487 GPU OC
Samsung Galaxy Note 8.0 - Stock
Samsung Galaxy Note 10.1 - Stock - Rooted - 1.6GHz OC
Nook HD+ - CM 10.1 (Retired)
Asus Transformer TF300T - Clean Rom 3.0.4 (Retired)
Acer A210 - Stock (Retired)
Verizon Samsung Galaxy Tab - CM 10 (Retired)
Barnes & Noble Nook Color - CM9 (Retired)
LG Optimus G Pro Stock...For Now
AT&T Motorola Atrix 4g -TopSmarts 2.3 (Retired)
nTelos HTC Hero - CM7 (Retired)