Is TXV required when an appropriate piston/orifice is used?
I believe he used an orifice, the correct size orifice will function fine in the system but the txv may be able to increase the efficiency. An orifice just meters the flow of refrigerant, the txv actually regulates the flow or throttles the refrigerant to maintain a constant temp in the evaporator. Yes a larger evaporator is quite common you would never want a smaller one but larger will be fine as long as it is not icing up. I have done both and had both.
orifice controlled less possible failures as 1 less active component,
TXV slightly lower cost to run but usually cost more up front.
If you wanted the txv in the system we would recover the refrigerant into a tank that’s a bit bigger than any bags I have seen, remove the orifice install the txv place the control bulb on the return side of the evaporator solder back up pump down and recharge the system with the refrigerant that was removed. It can take the better part of a day and I usually use the orifice on low seer machines (below18) and would in this case because your coil is larger you may not recoup the extra cost.