I'd look for a delta T between 16-20 degrees. Humidity levels will affect delta T as a higher humidity level translates to a higher latent heat load, thereby reducing sensible heat removal capacity. You will see the delta approach the 20 degree split on less humid days with normal interior heat loads and drop off toward 16 degrees with higher humidity under similar interior heat loads.What is the desired Delta t over the coil? It is hard to check it unless I drill a hole in the plenum.
Fan speed will also affect delta T. If you find that no matter what you do with charge and your orifice size is correct, and the performance still sucks, drop the blower speed one notch and then recheck the charge, delta T's, superheat and subcooling readings. Many air handlers come from the factory set at high speed in anticipation of being installed into crappy ductwork. Yours may have a lower static, therefore better flow, and you don't need factory high speed.
As for your TXV question, are you saying by "silver solder" you're using the same stuff a plumber would on water pipes? I would go with brazing on any refrigerant piping system, bar none. Some TXV's may have brass bodies with copper coatings, thereby the OEM may want brazing performed so you won't be wondering why the hell you can't get the valve to sweat up using anything else.