I don't ever recall anyone saying the center diff (aka viscous coupling) is ever at risk of "grenading" from mismatched wheels. However, as I understand it, you can trash your front diff - and probably never know unless you have it checked/tested.

A lot of good information in this thread, but (as I understand it) some misinformation as well.

Bottomline, the 996TT is engineered for a matched diameter (equal revs per mile F&R, or as close as you can get to it).

The 997TT is engineered for a mismatched F&R, with the rear being a taller tire.

So, putting OEM 997TT wheels on a 996TT is no biggie, but you have to switch the tires... running an OEM 997TT tire fitment on a 996TT will undoubtedly cause undue wear on the driveline, and most likely the front diff will take the brunt of it.

 

This article is bull. Typical tyre salesman pitch - scaring people. As long as tyres are N spec you can mix front to rear but I would keep same N designation per axle. Nothing wrong with N3 on the front axle and N2 on the rear. Bare in mind our cars came with N1 back in 2000 so anything above should be a step up.