In general, how is a TVSS sized?
It's funny, I'm wondering the exact same thing, nearly 4 years later. Near as I can tell, every TVSS on the market is oversized about 10-100x greater than the most powerful transients experienced short of a nuclear HEMP attack; I imagine it's just a natural byproduct of sizing MOVs for clamping voltage, and many places don't even mention the Joule rating. Oversizing by 10x does seem to increase longevity, though- an 8J module might protect against 1 bad lightning strike, but an 800J module will take them repeatedly and never burn out. So... so long as it's got the right voltage and KAIC rating, it's going to protect your system.
I have those IEEE reports, btw- the one most often cited is merely a report of measured transient reponses and more useful for people designing TVSSs than people trying to figure out which one to buy.