cumn thru,
I don't remember all the ramifications of moving the stab to different levels but the height of the stab has some bearing on rudder effectiveness at high angles of attack. I can't say much more about it without fear of dispensing bum information in the forum.
You may notice that some model designs use swept back wing leading edges for better knife edge flight. I believe there is a rule that says 5 degree of sweep in the leading edge is equal to 1 degree of dihedral. There are people in RCU who are modelers and have training in aerodynamics and I hope they can explain the technicalities in detail.

In the case of the UCD 60 and the 46, for me the UCD 46 was a better 3d'er right out of the box. The UCD 60 needs to have the engine thrust vectoring adjusted and you have to play with CG location. As you start to increase elevator deflection in slow flight the UCD starts to get squirrelly before the UCD 46 does.

As far as making modifications to the model's design, I don't even try that anymore. When I see a new model that I like I look it over, check out its specs and hopefully determine whether the characteristics I want are not outweighed by the bad.
When I first saw the UCD 60 I said "wow! Its a giant Topflight Contender" and I knew immediately it would be two handfuls during knife edge flight. I bought it anyway because I knew I'd like the other flight characteristics. The wings look almost exactly like the old Contender designed by Dave Platt many years ago. I built several Contender kits years ago and they were all difficult to keep straight in knife edge flight.

So, you'll just have to learn from experience how to look at a model's configuration and make an educated guess as to how it will fly.