That's the last resort: almost never you need to get that far(stracing/trussing usually do the trick), and it doesn't have to take "hours and hours" if you have done it before, specially if it's a locking issue. I don't remember if I have had to gdb apache(I think I have), but sendmail + milters for sure.

I don't doubt that your solution has been "battle-tested", but I assure you that apache + mod_php also has. Now if he has a different problem with that new setup what you will recommend then? Change again to cherokee or lighttpd? Or do you really think that nginx + fpm is a silver bullet?

I accept that spending some time cheap shooting randomly(raising os limits, changing threading model, upgrading to new version...) may be worth the time, though. But changing completely your middleware isn't a cheap shot in my book.