Hmm... From my experience if your shocks are blown, your car should feel mushier and bouncier.
Have you looked at your shocks yet?
If you see any oily greasy shiny stuff on it then they are probably going bad.
If the shocks are going, I would just pay a little extra money for a set of coilovers and get it over with.
Theyr supposed to perform reliably under extreme stress, so there should be no reason for them to fail early under normal driving conditions right?
Now about the life reduction of the stock struts you're wondering about, yes, increased stiffness does reduce the life of stock struts.
How significantly? I cant really give you a straight answer on that, there are so many factors that are involved.
Think of it this way. When you rub your hands together it makes friction and gets warmer. The faster and harder you rub them, the faster it is for your hands to get warmer and even hotter.
The same thing goes for your struts.
So when you take a strut thats only supposed to move a certain amount of fluid at a certain rate and force it to move that fluid at a much faster rate, it's gonna be making a lot more heat and stress than it used to causing it to wear quicker than what the manufacturer had said.
I hope I answered your questions and clarified any misinformation.
Here are the links of where I got my information from.
HowStuffWorks "Dampers: Shocks"
http://www.suspensionwholesale.com/A...l/Default.aspx