The Callisto is an excellent speaker IMHO, thought I'm not sure how it compares to the new Timepiece Mini that I've yet to hear.
I've not heard the Executive, but for the money ($3k), it should be
at least a very good value.
Better than the Timepieces? Dunno, but based on my experience with each brand, I doubt it.
My big picture take - SP Technology is far and away my favorite speaker manufacturer, but if they weren't around, I'd probably own GMA speakers. They come as close to the SP Tech's performance as anything I've heard, especially at real-world prices.
It doesn't hurt that Roy Johnson is a nice, down-to-earth guy. When you have to hand your hard-earned $$$ over to someone, it's always more palatable when it's someone you like and respect.
FWIW...