Okay -- I followed Lennon's procedure to calculate the lateral (spiral) stability of a model design. I was skeptical, because it sounded so low-tech ... but it appears to work. You find the "center of lateral area" (CLA) of a model using a 100% profile that is suspended from 2 or 3 different points, and run a plumb line down the length of the model each time. Where the lines intersect is the CLA. See Chapter 9 of 'Basics of R/C Model Aircraft Design' for the procedure.
What I found for the Sr Telemaster (ARF) suggested that reducing dihedral and/or adding to the vertical tail area quickly increased spiral instability. One of the posters on the mod thread complained that his plane was hard to get out of a flat spin ... dunno if these conditions are related.
Lennon says you have "neutral spiral stability" when the CLA is at 28% of the vertical tail moment arm (see diagram). My test suggests the factory Telemaster's CLA is nearly 30% of VTMA ... making it mildly susceptible to entering a spiral. When I took out dihedral (as a number of folks have done) on the mock-up, the spiral stability margin increased dramatically - to 37% ... well into the "very spirally unstable" zone in Lennon's estimation.
My intended mods include filling in the space between the LG wires (like a Cub), which improves the CLA location; and adding about 20 sq.in. to the rudder, which degrades it. I'm looking at a spiral stability margin of 32% ... still in the "mild" zone ... and I'll leave the dihedral alone.
Having gone through this exercise ... I wouldn't take these numbers to the bank, but the principle is what's important: if you move the CLA too far aft (even with the model balanced), there may be unintended consequences.

PJ