I must say I never used propertyGrids that much but in case o high frequent changes I would directly write to the dom and ensure (along with each writing or a trigger) that no events gets bubbled up to ExtJS.
Anyway, I am pretty sure that any change to the bound store will also affect the value. setSource
is just a way to apply data that get also inserted into the store. But this will always change all and I guess override any existing data (the last I don't know for sure)
But you can also use setProperty to change a single property. May that one is enough. Worth a try.