I did an RA IR a while ago and found it a great learning experience..
If you are happy to work part time on your weekends, then it will be pretty good for that too.
The school I did it with teaches GA and RA, and they made me do the GA briefs
Choose a school that has good standards in both prac and theory otherwise you may be teaching some poor sod a thousand different ways to die..
Knowing something and then knowing it to teach are two very different things, and it really plugged some of the holes in my knowledge particularly around aerodynamics...
Id say if you want to bring your knowledge along, and understand the principles behind what youve already learnt, then an IR is a good way to go.
It is also a good way to get into the air on someone else's coin, but it is a responsibility.
Down the track if you decide to do a GA I rating it lops 10 hours of the course(from memory), but thats about it..Also no matter what anyone sais here, if you have a certain amount of hours instructing RA, then you will be attractive to a GA school once you have passed your GA rating
Good luck