Hoppy,
In a much simpler response to your post, yes, you are correct that an H-tail increases elevator effectiveness through endplate effect. However, the size of the horizontal is rarely a driving factor in selecting this design (the wings are always much longer, right?).
However, endplate effect is one of the factors in using a T-tail. It allows for a shorter vertical, and if the tail is swept, a smaller horizontal as well since it places the aerodynamic center of the horizontal farther away from the CG.
And I'm not sure who brought up the F-16's twin tailed flyoff competitor, but it's also of interest to note that the YF-17 was a highly modified F-5 that eventually became the F-18 after some navalization (for lack of a better word).