Very professional work again.
Only I would try to use less reflection on plastics like the armrest or the wheels (maybe a drak grey could be nice).
You ask for noise and mxi. I have tested a white rectangle emitter and an emitter with a white MXI texture - no difference.
But you get more noise if you use much polygons on the emitter (a problem of MXI spheres) or strong contrast in small details (f. ex. HDRI from a full midday sun or a tunnel HDRI with two bright openings). Better is to use blur HDRIs, if your scene allow it.