I was out doing a spot of turfing in the garden and your problem was bugging me, more so after reading JMD`s reply..... I wondered if it might be worth taking it along to an equivalent of a local MOT station or tuning company to get it on their rolling road..? That way you might be better able to get in about things either while the engine is under load driving the roller, or without the engine running and the rollerr driving the rear wheel, thus simulating your road tests...? If you arranged a time you would be arriving with the engine warm perhaps closer simulating your tests....?
Might be best to ignore me, as I know nothing of M3W...Whatever... Good luck.
