Traffic boost is not normally much more than 4k in my experience. I've had two startups on TC (multiple times each) and worked at a third startup that also got coverage. Ranged between 3.5k and 7k uniques (so you're right-- it's on the low side).
Regarding investors, I think you might be right there-- TC coverage probably helped our YC app, for example (you'd know more than I!). But, in conversations with investors, I've NEVER had one say "Oh yeah-- I saw you guys on TechCrunch." It's a nice bullet point in an email/execsum that lends credibility.
Arrington literally has talked about founders calling him crying and begging for coverage and busting into his house to plead with him. That just seems wrong to me.