In theory it sounds great, but this qaffaz example, if is true, would be the exception (who buys those hides anyway?). I rarely if ever saw drops to be related to number of kills. Usually is on time waves. In which situation, higher DPP means less drops per ped spent. (considering the situation in which tt of MU item is higher than cost to kill).
But this is a side discussion.
Main discussion, applied to MU, is that there is no MU ingame to cover losses over a certain treshold. And my problem is that for me (maybe I am this shining exception, who knows) high efficiency combo, close to 70 eff, on as I said not longtooth, but a 500hp mob yelded TT return lower than 90% on aprox 4,5k ped spent. More specifically, I had when I left the spawn a tt loss of 540 peds. In order to pay off the tt loss and the MU of guns (the latter not being such a big issue though), I would have needed 6 drops of an item, (L), with a tt of circa 30 ped. It didn't happened not even once. On the very same mob the only time I ever looted that (otherwise a quite common item, not some uber rare unheard of thingie) was when I killed a few accidentally with 56 efficiency. An ul unimprovable item with, by comparison to ArMatrix guns, bullshit economy. Which item never let me down and on tens of K ped rolled with it, it never had the volatility and the tt losses I had with ArMatrix. It doesn't make sense.
To express it otherwise, maybe the above seems whining: I don't believe that higher efficiency should mean higher volatility. This is my main point of concern, coupled with the possiblity that higher efficiency might mean missing certain high multiplier, which would be necessary to compensate tt loss.