Like the title says, I ordered some Yokohama AVS ES100's off Tire Rack in 205/55WR-16. When they came I noticed they were Z speed rated rather than W.

What will the difference in speed rating mean for the tire, aside from the obvious fact that these tires are rated for 149+ MPH?

Is there a change in the rubber compound? Are they softer? Will they wear faster?

I am a complete tire noob, this is my first new set of tires on my RS after 48k of VERY worn RE92s. I can't wait to get them on.

Thanks!

Go read Tire Rack's tire faqs.
The short version is that "Z-rated" is a big set, which denotes tires tested over 149mph.
"W-rated" is a sub-set of so called "Z-rated" tires which denotes tires tested over 168mph.
http://www.tirerack.com/tires/tiretech/general/speed.jsp
You probably got the W-rated tires anyway, and not "Z-rated"

Ahh cool! Thanks!

the 205/55ZR16 has a 90W service description and it is a "W" rated tire