Nice shots! Love pics #1 & 3 especially.
I too am using the Canon 75-300 USM (IS version) with Uniplus 25 tube. Did you have any problems auto-focusing on this lens? Eg. the dragonfly in pic #2?
Oftentimes, I find that the lens is very s-l-o-w to focus and had to resort to using manual most of the time.