It's a nice composition, but as Bryn says, the dark areas have a significant blue tinge. I can also see a lot of posterisation artifacts in the background, so I assume this has been recovered in post somehow. I thought this might be a result of the compression from posting here so I check the largest size available on Flickr and they are still there. In the process I also noticed what looks to be a dead pixel just to the left of the "frond" (not sure of the correct terminology here) on the left and a couple of small dust spots just on the right of the right frond.
The extension tubes and raynox combo seem to be working well, what magnification do you get with that setup? There is some CA creeping in especially on the blurred outline of the first frond (everyone says this is easy to fix in post but I always struggle with it - I really must spend more time trying to understand how it works in Lightroom).
As for diffusion, I think you could try to get your diffuser closer to the subject, or if you are shooting some distance away (which may be the case at 122mm), make the diffusion area larger. It's obviously doing it's job and is much better than a bare flash, but I think you can take it further to give the lighting a more natural look.