I think the old REA (now RUS I understand) used to recommend no more than 5% drop in the service entrance. By "entrance" they were defining it as transformer (including the transformer) to the meter. If you have a 2.5% impedance transformer, at full load you have already lost half your allowance.
Considering that some primary line voltage isn't that great to start with, they may have a point.
It's a little off topic, but we've always had problems with customers not wanting that "ugly" old transformer anywhere near their pretty new house, so they insist on a 250 foot service drop. A week later, they're complaining about their lights dimming when the AC comes on.
