SFG mods do work - I had a Tribute and have a Tensor and a Showtime .
Ideally-- the plates simply add area at the CG you are using
IF--you add area forward the cg, the response in knife ege will become more sensitive.
When we did pattern models , it became obvious very quickly, lateral area placement changed knife edge flight.
The further forward the average of lateral area, the more balanced the lift in knife edge.
This must be weighed against stability in yaw .
The SFG additions are very high in drag-for the additional lift they add, but in foamies the loadings are extremely low and the angles of attack in knife edge are very low -so if you simply roll to knife -add no rudder - they can be setup to fly almost as flat as in level upright flight.
The Tensor turns with rudder -NOT from banking and adding elevator as many would assume.
Bank the plane --it just remains on heading-with wings tilted. Very strange visual.
just make up some plates and try em on the Tribute - the tail moment is way too long for my tastes so make the SFG's slant forward (try to ingrease lateral area forward.)
Ideally a simple increase of fuselage lateral area -forward -works better . more lift for far less drag induced.
This isn't from formulas -it is from flying em.