I think you are confusing the idler pulley with the supercharger pulley. The idler pulleys are used to keep the belt tight and are spring loaded and free spinning.
As for whether spinning the blower faster than it was designed, well yes that will wear it out faster. Our Eaton was designed with a 14,000 MAX RPM limit. The stock 3.65in pulley spins the M112 at about 13,9XX RPM when the engine is at redline (6500). There's a chart out there on the web somewhere that shows the different maximum RPM readings for different pulley combos. I'll see if I can dig it up.
Bottom line though is that anytime you take a mechanical component beyond it's maximum operating range you can expect it to fail or wear out quicker than normal. Just a basic mechanical fact.
No idea on the life expectancy. There are 03 Cobra's out there who've been over-spinning their blowers for 30K miles now without a failure. These Eaton M112's are tough that's for sure! There are Lightning owners who have been over-spinning their Eaton’s for even longer. It's not like you are constantly over-spinning the blower when using a smaller pulley. Only when your RPM's get above 5000 do you get into that danger zone. You asked in your original message if adding a smaller upper pulley would shorten the lifespan of the blower. The answer to that is yes, but compared to what? The engine? The drivetrain? If you are worried about wearing out the Eaton by occasionally over-spinning it don't lose any sleep over it! There are plenty of other problems inherent with adding a smaller pulley that will wear out your motor faster than over-spinning the Eaton! Primarily is all the extra heat that's generated. The Eaton M112 is notoriously inefficient. It generates a butt-load of heat stock and once you pulley it the heat generated goes sky high! If I were you I'd be more worried about adding some better cooling mods to the engine after adding a smaller pulley to help combat any future heat induced failures. Cylinders 7 & 8 are historically the ones to get damaged first due to heat because the cooling around those two is not as good as the rest. The additional heat generated from adding a smaller pulley also seems to expedite the wear of the Teflon coating on all the pistons too.
So the bottom line is your Eaton is not what you need to worry about when adding a smaller pulley. Combating the additional heat generated by the faster spinning blower is the real killer.
U.M.
P.S. I still have the stock pulley because I value my warranty! Once my warranty is up all bets are off!