Paring up an aftermarket spring with more aggressive spring rates will definitely cause your OEM non-sport or sport shocks to wear out over time. This is due to the fact that OEM shocks are simply not meant to handle the more aggressive spring rates. Think of it this way, you're putting a shorter spring with a longer shock body. The shorter springs will cause the shocks to compress to its maximum setting every time you hit a bump due to the longer travel distance. What this can cause is what's known as bump steer and jarring of the ride. This is what for the most part affects ride quality.
Let's break this down in terms of cost. You'll spend about $230-300 bucks for aftermarket springs, another $250-300 for an installation, and another $65-$125 for an alignment. When your OEM sport or non-sport shocks blow your going to have to spend another $700-800 on shocks, another installation fee, and another alignment. If you calculate that all together, you would saved a whole lot more money doing it right the first time around.
With that being said, I would most definitely recommend that you go with a coilover system and do it right the first time. The answer is simple; you get what you pay for when it comes to vehicle suspension. As such a crucial component of your vehicle, suspension is definitely something you do not want to cheap out on. A coilover suspension basically gives you a matched lowering spring and shock absorber combination with the added benefit of adjustable ride height and spring rates through a moveable spring perch or perches. Coilovers are to your car's advantage by reducing unsprung mass, increasing suspension travel, and allowing the car to be corner balanced. With that being said, I would recommend for you XI aka X-drive guys to check out KW coilovers. KW does make both the KW V1 xi and KW V3 xi models! Here are a few pics of the KW coilovers drop for you guys to check out!