Edit...comparing the YS53 photo against the early T53...looks like they increased the fin area quite substantially so presumably the early T53's had directional instability problems !!
I operated a Slingsby T53 as Reading University project pilot on meteorological reasearch 1970/72. I recall (and my logbook appears to confirm) that it soared surprisingly well, and the handling, albeit rather heavier in roll than a Ka13, was fine (including sufficient rudder for full aileron, and full spin recovery at aft CofG). I don't know the reasons for the Yorkshire Sailplanes mods, but was not impressed by the build quality of the example I saw.