Okay a couple of thoughts.....
Over the years Futaba has done their fair share of messing with things which produce undesirable results particularly with other vendor/manufacturer's hardware and the signal line voltage is just one example, the other real common one was the sinking current on the R149DP rx's which would not allow you to run Hitec servos on a Y-lead.
That said; Hitec adapted their servos to run on the lower signal current of the R6014FS Futaba rx's, Futaba came out with various upgrades to that rx which included hiking the signal current voltage and those rx's are signified by first a white dot, then a gold dot on the sides of the rx's and finally the release of the 6014HS rx.
Second thought is that there are guys using the big Futaba servos and reporting good results but as you noted they are not cheap. There have been some issues reported with the Hitec 7950's running on un-regulated Li-Po voltages and long servo extensions but that seems to be pretty much isolated to the Spektrum/JR PowerSafe rx's for some reason not yet identified by Hitec. Those reported difficulties do not seem to affect the 7955 Hitec servos in similar installs.
Last; if you are looking for servos that big, it would suggest you want to run a GS gasser setup? If so, may I strongly suggest you do not plan on running these size servos on Y-harnesses especially reversing type Y-harnesses, verify that your rx and battery system can handle the amperage needed to run these big servos and pay attention to all of your electrical components like switches, wire guage sizes, connector types and available current paths into the rx.