You can flash back to stock anytime - the unit saves the original file

 

It is possible to do the JZ swap and there is some info going around. The best info i can give is to contact Tweak and have a discussion with them on what your coal is
and let them build the main harness for u. U will have to run duel ECU's because of the CAN system of the IS. The tricky part of this build is the communication part from the ecu's to all the necessary modules and electronics. It is just a case of to have all the relevant info to make all parts to communicate together or trick to communicate to each other. If someone out there that already did the conversion can give more info for the electronics side of this build it will assist a lot of people doing these swaps.

 

Personally if you want to have power i would suggest getting an is350 or isf. an is350 is a couple grand more but youll be spending more if you supercharge an is250. Although those mods are a good way to get more power i just think its too expensive when you could sell the is250 and buy an is350 for a little bit extra