On a side note, I've managed to teach my orchid mantis to take wax worms, so it can be done even if it is difficult at first. I usually use a pair of tweezers to squish the head of the wax worm, then hold it to the mantis' mouth, as is customary when hand feeding. At first, he was quite scared of the wax worm and didn't know what was going on, so it took quite some time to get him to grab it. Months later, he'll recognize the procedure and grab them pretty much instantly.
About the fattening issue, how does it affect insect metabolism? For herps, fish, mammals, etc. it would be easy to figure out but insects are different. I mean, some of them eat almost nothing but sugary liquids, a diet that would mean death to a herp, fish or mammal. Also, is the effect different on an adult as opposed to a nymph?
Secondly, I've noticed that at least my wax worms seem to contain different liquids; one transparent one that looks like nectar and a white thick one, which I suspect is more fatty. My mantis typically seems to like the nectar-like fluid best and then discards the remains of the larvae when the good stuff has been eaten.