Is that a correct extrapolation of the maths?
Not really. This is a nonlinear, overlapping probability problem. You can't just make a hand-wavy multiplicative extrapolation. Note that the math KD did was only a couple of isolated cases. If you want to get the real picture of how to make burst weapons equal to other enhancement, you're going to have to run a full analysis of their effects at all cases of chance to hit, chance to critical, and amount of critical damage. I wish I had the time to do it myself, but I'm at work (and tend to be lazy over the weekends).
Pet peeve: I really hate it when people use the word "maths". But I understand you're British, so I guess I can let is slide

For comparison. On the one side, d10 lightning, cold, or fire damage. On the other side, DC 14 deafening, d8 sonic. The sonic type is almost worth the drop in damage alone, and then the deafening thing is added in.
I don't think this comparison is very fair, either. The burst enhancements are slightly underpowered, but I think the Thundering ability is also slightly overpowered (and pretty campaign dependent as well). I see both as being more for flavor than crunch, anyway.