I'm getting to be quite enthusiastic about the 300mm PF + TC1.4 II as a close-up lens, as long as the subjects aren't too small. Here's one from today of a small white on the lavender, that I was able to shoot much more comfortably than creeping up close with the 90mm macro lens.
I'm not sure how far I can push it, but here's something smaller, namely what I think is a wasp but my brother-in-law (in whose garden I photographed it) says is a honey bee. If it is indeed a wasp, that's another plus for the extra distance of the 300 combo .
C&C on the images, the wasp/bee question, and on the idea of using what was bought as a birding lens to take insect pics are of course welcome.