My question now is "what is (or are) the missing factor(s), especially in regard to speed control?" Nail mentions the "smooth powerful boot drive/edge set at the turn finish" followed by the release of stored energy in the flexed ski (or "rebound at the turn finish/cross under to float into initiation of the next turn" as he puts the point) into the next QCT. He also mentioned the importance of reaching further downhill, which will – as he puts the point, "gain you more shovel pressure earlier on as you drive down the fall line" in steeper terrain.
Seriously, after reading that post, I'm thinking Father Time himself must look down on you with envy. Challenging Black Diamond runs, yet alone attempting to hold a tight fall line at 70? That is a STATEMENT and I feel honored to be even considered for guidance.
I can just see the grin on your face as you hang in the thin ribbon of 10" fluff on the edge. I'm assuming the QCT is a tighter turn than your used to consistently making, the more you practice it and experiment with variations of it, the more confident you'll use it in varied conditions/pitches.
Speed Control and how to gain it.
Let me first suggest reading the "Mogul Skiing" thread if you haven't already. Lots of very important and pertinent information there if you can wade your way through the pissing matches. Pay special attention to the posts that discuss the "Technical Line" and stuffing the tips, also try to visualize the references to fore/aft movements/shin drive/boot drive at the turn finish.
I've never skied the Gotama or the AC 30. It's funny, but a very good friend of mine that I learned to ski with has the same 2 ski quiver, but skis the Gotama almost exclusively, he's skied at Snowbird for the last 25 years. Since I've never skied the "Goats", or a rockered ski yet, I'm not sure how the rockered tails brush at the bottom of the turn into the turn finish. This is where a lot of speed control can be gained.
It is important to complete or finish the turn in order to dump speed. If you finish your turn to early, drive/pressure your feet slightly forward while on high edge angles to early, your skis may cross the fall line at let's say a 45 deg. angle to the fall line. While this angle may have been sufficient to harness enough rebound energy to float you into the next turn transition, you may not have dumped enough speed/energy by completing the turn to early. If you hang or delay the turn finish slightly, your skis my get to a 70 deg. angle to the fall line (skis at 90 deg. being perpendicular to the fall line). This slight delay of the "sting" (boot drive/pole plant) at the end of the turn allows the ski tails time to slide/brush and dump speed. This is what we describe as the sensation of a round turn, it almost feels like the skis are crossing the fall line at a 90 deg. angle when they cross under you.
When you reach the pole down the hill it will get the body further forward over the shovels, this will naturally allow the skier to put greater pressure on the shovel edges as the skis near the bottom of the turn, they really start to bite and come around. As the skis start to come across the fall line, the skier wants to smoothly and firmly shift the pressure that has been focused on the shovel to the tail of the ski. Fore/aft movement. One way to facilitate this shift of pressure from the shovel to the tail is to close the ankles while slightly opening the knees, driving the boots slightly forward and rocks pressure to the heel. If you need to dump speed, hang in the bottom of the turn longer. Close the ankles slightly slower and open/extend the knees slightly slower, you'll feel the resistance. This will allow you to hang or delay the turn finish slightly allowing your skis to get more across the fall line, all the while dumping speed/energy as your tails brush across the snow.
Remember, these subtle timing delays and intensity of movements can result in big changes to speed control and roundness/shape of the QCT. Also try to remember the sensations you had making QCT's in the softer snow and try to reproduce them when it gets firmer, those are really the feelings you want to emulate when the snow gets harder.
BTW, if you could get any video, it would be great. Bumps, groomed, crud, or powder, it doesn't matter.