Actually your question was about the speed rating affecting tire life. Tire life is determined by the compound. A harder compound will last longer. Just because a tire has a high speed rating doesnt mean it wont last. For example my Kumho Ecsta ASX tires are ZR speed rated but are 205/50/15 which isnt that low of a profile. They have a treadwear of 420 which is quite high, and still have decent grip.