I've done thousands of pictures, mostly insects, with this lens and was never bothered by CA but I'm indeed almost always around f6.3 or f8, at least, to get some depth of field. AF is quite handy to get a fast focus that I refine manually when needed. Optically, the old Apo Lanthar is much better, but I can only use it with quiet subjects.
All in all, I find the AFS 105/2.8 a good workhorse, not brilliant but reliably good for outside macro of insects.