I needed a different stock for mine due to barrel profile (would have had to cut off a ton from the factory stock), they are nice an stiff though and depending on which you pick change the ergomics of the grip, cheek weld, etc. They "drop in" if using stock bottom metal but any new stock might require minor fitment and also benefit from bedding. What specific "upgrade" kit are you referencing. I've seen various. Some change the lug out (the T3x are already steel), some change the shroud out (the T3x are metal now if I recall right but I never cared about the plastic one on my stock T3), some change the butt pad which is a comfort issue, etc. Most improvements and whether they are worth it are personal preference. My personal preference is a premium barrel with a well cut chamber is likely going to shoot tighter that an factory barrel which may or maynot be useful for a given person, also it allows one to use non-factory calibers if desired. Think about what you want in a finished rifle and work backwards.
. 
