I'm just getting into MODing my "S" and was just wondering at what point I need to consider upgrading my Fuel Injectors?

 

Depends on your goals.....
First off many people run a 15% reduction with a CAI, and cat back, no problem...
But....When a custom tune is done, it is found that more fuel is needed to get the last few HP out of the motor at high RPM's. (think about it....there is less time at high rpm's to inject the fuel because the valves are open for less time). The stock injectors are generally considered "stressed" with a 15%, and can sometimes run a bit lean with the 17% at high rpms...and the above mods by most reports.
The thing to remember is that bigger injectors REQUIRE a tune to run optimiumly....they must be scaled so that the computer can control them....
JCW's 380's can be run with out a tune.......BUT the car will run richer than optimum at throttle settings beyond those that are adjusted bu the O2 sensor..so called WOT. But you should not have caution lights/engine codes getting thrown.
Go bigger than the JCW's....and you will need them tuned to run...preferable at the tune site.
If you want to make the best power, as efficiently as possible, and you do any real motor work (anything beyond a simple bolt on)....a cam, etc, then bigger injectors are really needed to ensure long engine life.
To put it in perspective...many folks can make just about 190-200 HP with the stock injectors,(and the above mods...Pulley, CAI, Cat-back) and they can get maybe 10 more from bigger injectors...since they resolve issues of running lean.....
Which ones to get depends on your goals...if you just want to do simple stuff...like above...the JCW's are fine....if you aspire to do a cam, etc, go to 440's or even 550's...they will give you more "head-room" for future mods+tunes.

If you do a quick search....you will find tons of info!! Good Luck!!