This does get confusing, because we are talking about dosage not only in mg per kg (or mg per pound), but also in ml actually given. The key is the concentration (number of mg per ml). Your compounded Bupe is unusual in containing .5 mg/ml. Bupe is normally supplied in a concentration of .3 mg per ml.
If I understand your post correctly, not only has the vet increased the concentration from .3 mg per ml to .5 mg per ml, he has also increased the dosage from .2ml of the lower concentration to .5 ml of the higger concentration. Am I reading this correctly? If so, I would really want to check this with the vet.
A higher dose of Bupe will provide a longer duration of pain relief, not a stronger level of pain relief. A higher dose will, however, also cause more side effects (increased sedation). Giving .2 ml seems like a good compromise until you can discuss this further with the vet.
When you say you are giving this orally, you are giving it in the cheek pouch, correct? It is meant to be absorbed through the mucous membrane of the cheek, not swallowed down the throat.