Good one mate. I was going to comment on the highlights from the flash, but didn't. Not sure if you've played with the exposures at all in post-processing. Assuming you haven't, I thought you handled the background exposure very well. It's always nice to see macro photos with some colour in the BG. Quite often, because of the use of flash in ETTL mode, macro shots tend to have bright subjects and black BGs. Sometimes that's intentional and effective, but more often than not I find lit BG more appealing.
I think you're not far off the mark with your -2/3 under. ETTL flash is a bit finicky and random, and agree the metering mode may have something to do with it. Don't think spot metering is the right way to go, though. The area is covers is quite small, and given how much variability there is with moving subjects I think it would be difficult to get consistent exposures. I'd stick with, and experiment with, evaluative and average metering.
As you probably know, the closer you get the exposure right without the flash, the less the flash has to work. What we want is fill flash effect.
The other factor is that both the MT-24EX that I have, and your MR-14EX, are known for giving pretty harsh light if they're not diffused. The flash heads are very close to the subjects. Don't know if you noticed what I was using for diffusers - I have
Lumiquest Ultrabounces on each flash head and like the results I'm getting even though they're a little big for my liking. Might try cutting them down somehow.
I'd do a google search, or maybe have a look through
POTN's macro discussion section, to get ideas of what other guys are doing to diffuse them. Might be a little difficult on your ringlight given the shape of it.
Hope that helps.