JonnyG -
Welcome to EM! The SG is like a bio-feedback device for your car. Since you're already doing good with your Rav4, you may not see as big a jump as others, but you should still see an improvement. I always emphasize that the SG will also give you error-code info and portability. You can put it into other 1996+ cars.
Because you said 24 MPG highway, I am assuming you have a 4WD. Therefore your new-EPA combined MPG is 21. If you are almost at 25 MPG, then I think you could get another 2 MPG. Maybe that's low, but I am assuming you have already mastered a lot of techniques.
Here are some other Rav4's in the garage for you to look at :
EcoModder Fleet list - "Rav4"
CarloSW2