I'm sure you figured this out by now, but for anyone else looking to purchase these tires, here is what i found out.

Pirelli Scorpion Ice & Snow - 255/55/18 109H M0 (Mercedes) - 14"/32" Brand New
Pirelli Scorpion Ice & Snow - 255/55/18 109V N0 (Porsche) - 11"/32" Brand New

There is a diffference of 3" when comparing the 2. I would think this has something to do with the speed ratings being higher on the Porsche than the Mercedes (H vs. V)

It's not a difference of 3". It's 3/32". That's barely perceptible. Two different speed ratings, two different tires.

Yes but if you figure 3 or 4 winters out of a set, that's almost another winter season difference. Either way it's good information.

again, not really. two different speed ratings may very well be two completely different tires. the compounds, wear rates, etc... will likely be very different. if you are going to compare, find the same tire but in different sizes. then the two should match, but you really cant match speed ratings against tires on something as granular as tread depth.

Was just trying to provide information that most people are not aware of.