You can find the injectors that come with the magnuson for sale all the time on this site. As most people pull them out with VERY little use or never even use them at all. Look under the Members Classifieds - V8 parts also under Forced Induction. Yes the ID850 are the top dog. But it's your cash and for $200 or so you can pick up some injectors that will get you to 500-550rwhp just fine.
People will beat you to death for it and tell you to just get the ID850s now because you will always want more power so why not be ready for it. But in the end your the one paying the bill.