Probably not much of a diet. If you were to remove the engines and diesel tanks from the centre vehicle, you'd be changing the "dynamic envelope" (how it behaves through pointwork and around curves, or when buffeted by air circulation when passing other trains, going under bridges/through tunnels, subjected to crosswinds etc.) so you'd have to redo all the route clearance work. Or, you replace the engine and diesel tank with weights (as per those DC units that don't have dual-voltage capabilities so have a concrete block in place of the transformer) which defeats the purpose.
In any case, making one vehicle out of three light enough to use differential speeds is pointless when the other two vehicles still can't. If they're moved to a less challenging route you can make more liberal use of the "eco mode" which shuts down one engine when the full power output isn't needed.
Replacing all the interior fittings with lighter ones would have some impact, but probably barely enough to scratch the surface. I suppose you could in theory do something really radical and replace the bogies with inside-frame ones to save weight, but that wouldn't be worth the cost.