Don't worry about this too much.
I wanted to point out that there are 2 different kind of "speeds" to consider: one is the reaction time of the radio system - this is the part that HRS supposedly improves (I personally notice no change when going from HRS to normal receiver. I guess I'm just not fast enough myself
). The other is the servo transit speed, this is the part that you set as "servo speed" from the Tx. HRS or no HRS makes no difference here (if servo speed is not limited by settings).
I don't know if slowing servo speed works in HRS mode, but from the Futaba answer you posted, it looks like the answer is negative. But methinks this is not a good thing to do anyway
I know you didn't ask _that_, but whether you actually _need_ 3PK and all it's bells & whistles is another story. I've been racing for >10 years and 99% time I only use steering & throttle EPAs, nothing else. I'm very happy with my 3PK, but it didn't make me any faster than I was with 3PDF. If you're a newbie, there are better ways to spend your money, (tires, batteries, charger and motors to name some).