temperature and voltage are the two primary factors in a chip or memory sticks' lifespan. changing just the timing and mhz isn't gonna do a whole lot of anything.
This is not technically an overclock; you have just tightened the timings.
As for the memory's lifespan, it depends how much voltage you have put.
I guess you didn't have to push the Vdimm too much to reduce timings without increasing frequency. In any case, it is always good to keep the momory sticks as cool as possible, making sure you have good airflow in the case, and with hight volts, that you have a fan blowing directly on them.
then you're not overclocking, because by definition overclock is exactly what it sounds like: raising the clock speed to a value higher than stock..
changing the memory timings would simply fall under the "tweaking memory" category.. while not overclocking, it can still boost performance. though don't expect any miracles, memory speed and timings do very little for performance outside of synthetic benchmarks