They will lift above the strut tower a little, rather than sitting directly on them as they should. Only reason I could tell on mine was from hitting something hard enough to bend my front passenger side wheel and bottom out the suspension on that corner. It shoved that upper strut mount up almost a 1/4", definitely noticeable visually. (Though I drove around for a month before I noticed it, caused my front tires to wear a little weird).
The upper strut mount shouldn't require modification for coil-overs. I think I have about 40,000 miles on GC's, the one I didn't break is still fine. I'm not sure if they really wear out faster, but the risk of damage from a pothole or whatever is obviously a little higher.
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