Perhaps the stock RTR HB diffs are weak - they are similar "pot metal" as Serum calls it. I only carry the hardened gears and only used the HB diffs with the hardened gears - these are as strong or stronger than anything else - definately stronger than a Kyosho non hardened gear without question.
Also, Squee - HPI/Hot Bodies were always related - they moved into the same bilding last year or the year before, but were always related. I have ordered gears before and after the switch and nothing changed. I think you may be thinking of the non hardened gears(which will be considerably weaker regardless of the manufacturer).

Yep, these are the hardened gears i used for my savage as well. they rock.

And will simple outperform a UE 6 spider in terms of strenth(since the 6spider also uses these weak aluminum gears) (the 8 spider uses the same aluminum for their gears)