Yep, sometimes following the manufacturers recomendation results in buying something else they produce and are trying to promote. I bet if you go with some $150 whizbang JR servos, you'd be good to go.

But seriously, knowing full well that anyone running Spektrum/JR radios will be using 6v packs, I think Horizon should recommend a servo that won't freak out with a fully charged 5 cell NiMh pack, which can peak out to closer to 7v with any decent charger.