Dagnabbit, I came in here to write what Ron said... too slow!
But yeah, the short version of a long physics-based argument is shown in that video, starting at 0:38.
While I'm certain that a wide-stance leaning reverse trike can be made pretty fun to ride
with the proper suspension and (likely) electronics, I don't think that's the answer right there.
That said, I *do* think there's promise with the narrow-stance reverse trike design, ala the MP3 Piaggio. I want a motorcycle version of the MP3 vice scooter-- you'd still have to ride it like a motorcycle vice trike, but you'd have at least a little more stability underneath to deal with those nasty urban environments and road surprises.
As for the other issue, getting the most out of your Spyder: with practice and the right setup, you can take the Spyder through turns *quite* fast. My rules of thumb:
1. Fill up those tires (I ride 18/28, that makes a difference)
2. Get the better swaybar, better shocks, or the better swaybar AND the better shocks.
3. GET OFF YOUR BUTT! Riding the Spyder to the fullest is a physical activity. It's a great workout, yoga of the first order. You want to slide off the seat *into* turns, lean forward towards the mirrors, use your feet to push off the pegs, and use your inner thighs to press against the body.
With practice comes confidence, and with confidence comes the knowledge that you're much likelier to scare *yourself* off the turn than uproot the Spyder, especially if you're hanging off the side drunken-monkey style.
Now, I'd like to add somewhat paradoxically, that I found riding like this to be more useful *in the very beginning* than I'm riding now, with close to my first 2,000 miles in. Meaning, when I first started riding I found it easier to exaggerate my movements and leaning to help take turns at speed. Now that I'm growing comfortable with the limits of my RS, I'm better able to judge the speeds and corners I can take without hanging off the Spyder.
(Alas, every so often I still come into a turn a *wee* bit too hot, and only realize at the last second that I need to hang off
even further... hey, I'm *still* new to this! :doorag

.
Anyway, the Spyder isn't a sportbike. But it's definitely more nimble than a cruiser, and competitive with many touring bikes out there. But there's no reason you have to ride it all crazy-like: you can do sleepy sweepers all you want, and the Spyder will enjoy itself.
P.S. I want a Spyder racing series, somewhere, just so I can see how the best Spyder riders maximize this machine. BRP needs to sponsor something to show off what this thing can
really do.