You hit the nail right on the head when you said that not having to think about your casting motions with a given rod is what makes casting pleasuarablle. That is why I like stiff, fast recovering rods, I don't have to think about what I'm doing when casting them; whereas, with the moderately stiff or soft rods, I have to think constantly about the need to use a longer, less quick on the power application stroke. Like you said, "When everything matches up, it becomes effortless".
Even with the extended-belly lines I use the same stroke, I just increase the amount of force added and extend its length just a little. This still applies a lot of line speed to all aspects of the cast, including the upstream sweep and D Loop formation. I also use both hands and arms pretty much equally to apply power to spey casts (including when forming the D Loop), and this really adds power very quickly while retaining the shorter, more explosive stroke I prefer. I must admit that a single spey's anchor can hit the water with considerable splash when using this style of casting stroke with a stiff rod and extended belly line; however, the large splash only occurs on cast over 80 ft with a floating line so it effect on fish in minimal.
Like you, I also down size my line with the extended belly lines. In fact, I've found that I must use one size smaller GrandSpey than XLT on a rod for it to feel right and not overload the rod.
You are absolutely right about stiff not being the same as fast. Stiff is resistance to bending, not rate of recovery to original state from bending. Take a hook for instance, it is stiff, but it has lousy recovery. In my mind, fast refers to the recovery rate of the rod after it has been bent under a casting load. And you can have a fast recovering rod without it being stiff. A stiff rod can also be progressive and not tip actioned, it does depend on how much power is applied, how quickly though.
Your observations about fast vs slower when describing a rod is why I've taken to describing rod action using stiffness, rate of recovery, and power instead of fast, medium, slow. Using the three parameters of stiff, recovery rate, and power gives a far more accurate picture of what the rod is doing under a casting load.