Lowering usually will increase the camber a bit and hence can cause the inner side of the tires to be worn out more. You can do realignment to minimize this and in most cases it will not cause too much excessive wear compared to before lowering. Most cars on std condition should have <-1 camber. With lowering it can reach -1 to -1.5 which is still ok for street use. If after lowering your car camber even more then you may need camber nuts to help bring it back.
If your car hit those speed bumps etc. in the under body, it should be ok. Most of the the time, it's the muffler/exhaust pipes that get's hit and can break but only if severe case. What you need to be more careful is usually the front lip spoiler/bumper of your car which often can get scrapped when going up/down on ramps in/out of malls.
In terms of absorber, it depends on the sport springs that you use. If the spring rates are very stiff/high compared to std then you may need to change the absorbers as it's not strong enough to dampen the spring movements. But there are many sport springs which are still compatible with std absorbers too, for example Tanabe NF series. Of course there's still effect to the lifetime of the absorbers as it definitely has to work harder to dampen the slightly stiffer springs but in general should not brake the absorbers so fast. Maybe just reduce the lifetime a little bit only.
This post has been edited by 6UE5T: Dec 1 2012, 06:05 PM
Lowered myvi 1.5