Are shift kits good or bad......The answer is both/it depends. Firming up the shift will reduce the amount of clutch slippage which will reduce heat and help save the fluid (as well as less wear on the clutches), but as in everything, moderation is the key. Depending on what you have in front of the tranny, and or behind it, a shift that is too quick or too hard can affect other parts of the driveline (u-joints, rearend, tranny interals, etc.). Hard shifts can be tough on sun-gears, input and output shafts, etc. and can actually lessen the life of the tranny, unless other parts are "beefed" as well.
A Mild shift kit (mainly pump and valve body modifications) will help just about any tranny, but when you start getting into "heavier duty" mods, it will depend on the rest of the components as to whether or not it is good.
For a Stock 700, I would say go mild and add a good cooler, and possibly a remote filter. They are a good tranny, but not the best (especially in stock form). Also the year of the tranny (700) can make a big difference. Doing a search here and online will give you some good info on which years are better than others and why (mostly input shaft and valvebody differences).