Z is also a very vague speed rating. It has two different meanings. For years, Z was anything above V rating (V is 149mph). Z was just written as 149+. Then came W (168mph) and Y (186 mph) and the new Z rating was used for 186+ mph. The new Z rating, if the tire company is nice, will be written as ZR instead of Z. So if your tire says Z, they're either high speed race tires or more likely V rated tires marked to look fancy with a Z that doesn't mean anything.
Also, speed ratings are mostly meaningless dribble when it comes to tire performance. Higher speed tires have more sidewall stiffness so they don't explode hitting bumps at their rated speed. Your car is transmission and aerodynamically limited below V speed rating. The tires I use in the winter are only H rating. They "only" lets me go to 130mph. Tread and compound are much more relevant to handling performance than speed rating.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD