(Sigh... I posted a nice response, then my connection burped just as I tried to save it and it went off into the ozone... I hate computers. I'll try again...)
Shift kits: Generally work by making the transition time between gears shorter. Smooth upshifts are the result of one gear being slooowwwwly released while the next gear is being sloooowwwly engaged. The transition time is controlled by valves/orifices in the transmission that control hydaulic pressure on the various bands and clutches. During the transition, both gears are partially engaged and everything is slipping. Shift kits speed up the transition and make this process more rapid and abupt, hence the harshness. Good for racing (and trans clutch lifespan), not so good for comfort.
I've got a TransGo kit in mine that has three different calibrations, all increasing in firmness. I'm running the "middle" calibration right now. With the converter and gearing I'm running, at part throttle it's actually difficult to detect when the upshifts happen. I haven't had any experience with full throttle operation yet, owing to a tendency to toss the alternator belt when I hammer it (that I'm working on solving), but I suspect the experience is going to be "exhilerating".
I can tell that when engaging forward or reverse from neutral/park it does hit a little 'harder' than it did, but it's not too bad.
Some kits (including mine) also provide the ability to hold any manually selected gear "forever" regardless of rpm, and likewise the ability to downshift into any gear at any rpm/speed. Again, useful for racing but obviously you're got to keep your head screwed on with this one. Selecting low at 70 mph would be "not good".
Like so many things with these cars, it's a compromise... do you want smooth shifts and docile street manners, or do you want max performance / lowest possible e.t.'s.