I love bieng a "red shirt". I would simply provide this caution: I became an SFI because (1) I love to shoot and (2) I believe I have the capacity to teach. Soon after becoming an FI, I discovered that it worked against reason #1! I don't get to shoot nearly as much because I am always running the line, not standing on it. Now, when I want to shoot, I have to go out and do it on my own. Sadly, I actually shoot a lot less now that I am an FI. Keep that in mind.
Of course, the great thing is that I get issued one of every gun in the inventory.
Our agency requires a higher demonstrated level of proficiency (85% for FIs vs 80% for other armed personnel) for Firearms Instructors. Additionally, there is a separate course of fire that must be passed at the first day of FITP school and potential candidates must demonstrate the ability to do so before we send them. We had a rash of guys failing at FITP (on the first day, no less), so we are much more careful now. Additionally, Senior Firearms Instructors review the perspective candidates for their ability to teach and interact with shooters. Not all great shooters are cut out to be instructors.