According to the "Transmissions 101" series of articles (under Tech Articles/Transmissions at TCCoA.com), the ATF/filter should be changed out every 15 to 20k. The owner's manual recommends 30k but it also recommends 7k for an oil change!
Mine was changed at 40k along with the jerry-mod and it was rather dark. My valve-body gaskets were also baked on and crumbled when we removed them.. (not good).
The harder you drive it (the hotter the trans tends to get) the sooner you should change it. I got it at 17 and had probably 150 1/4 passes on that oil
Chip shift kits do nothing but boost line pressure and add stress to the transmission. The j-mod widens the opening for the fluid to travel through instead of trying to force more through a smaller opening.
DEFINITELY do the j-mod. Use the lightest specs too, its plenty of kick.
i usually go by time and not mileage for transmissions. i flush the trans and change the filter every year. the next time i do the filter im thinking of getting an aftermerket pan with a drain plug, im tired of getting a ATF bath when i drop the pan.