Will a 20 mm offset rather than the 42 mm offset wheel cause any issues on these cars?
It would put the wheel about 7/8" further out, is that going to make much difference to hub longevity?

Never seen an issue going offset like that before. Usually the issue is going the other way and sometimes the wheel rubs the strut, or the rim rubs the brake caliper. Or depending on the type of rim/tire, you could hit the lip of the fender well.

Thanks for the answer.
Where I'm at is trying to find some suitable winter wheels for the winter tires, I'm in Canada, Eh!
What I have a shot at is some aftermarket alloy 18 x 8 wheels with a 20 mm offset and like to think that once in a while I have enough sense to ask for input.
A bit of "look before you leap", I'm tired of the hard landings!
The winter tires are 235/50-18 and have been on the stock rims before, I can't see that there would be much issue with that little bit more of offset, but it never hurts to ask.

With more offset, you may feel like your riding on rails in the corner.

I've been pretty much all over the net and there sure isn't a lot of info on these GXP'*, nice to be part of such an exclusive group! I guess, as long as you aren't looking to the masses for education!
The more I get into playing with this car the more I realize that they aren't the car you see everywhere you go.
That being said, I have 1983 Porsche 928S, no, you don't see them everywhere but if you check out the Rennlist forum it'* shocking how many daily posts there are!
I guess the good old domestic big muscle GXP must outclass the poor little German V8 muscle!?!?!

GXP hubs are a known issue, but 20mm....meh. Could be tight in corners as Mike mentions.

I run the OEM GXP wheels with Blizzaks in winter then switch out to Moda M5 chromes with Conti Extreme Contact DW for summer.
Like Venom said, hubs are an issue, knowing that years ago, I chose to stay with OEM offset