Hmm that's food for thought. In that scenario, would you expect it to cause permanently high oil consumption, or only when running on petrol? If its doing it temporarily as a consequence of disrupting the injector spray pattern then I would expect it would not burn oil when running on LPG (on the basis that the LPG doesn't wash the bores).
Given that it uses oil regardless of which fuel it's on, but only at low rpm, I wonder if it has been washing the bores when running on petrol and disrupted proper ring seal? That would be bad as it'll do it to the same to the new engine
That said, dodgy rings doesn't really correlate with the old engine being able to hit vmax. Wouldn't it be low on compression in that case and so also be down on power?