only thing i'd advise against using the stock rims for snows is the fact that the rims are staggered. 7.5 front and 8.5 in the back, thus you have to have staggered snow set, meaning you can't rotate the tires every season, ending up w/ shorter life for the set. (altho some may argue that its not enough to get an extra season out of it.. who knows.. i'll try and let you know in about 3 years
)also i don't think you want to go any wider than 225s for snows. for a while i was gonna get 205s, but potential change in ride characteristics made me stick w/ 225s.
m68s are nice looking rims, in fact i bought a replica version of it, (7.5s all around) for my winters. my second fav rims after 135ms.(altho i'm not sure if it exactly looks stock.. for some reason stocks look better than the ones i got. i'll have to take a closer look at the oem one next time) as for perf gains w/ ligher wheels, i don't know much about it, so i can't comment on it. but they would be prolly be much cheaper than getting oem 135m rims, esp around $500us a pop. altho there is a replica version out there. a friend of mine picked it up, i can ask him where he got it, and the pricing on it. i think they were 18s.. not sure tho.