Hi Johnny, my bike but Mike Feltons job (who has just set up his own VMX business and may produce replica RH frame kits), and I agree he made a superb job. Tank is TM125, seat gap is the same as genuine RH73, there are a few RH bits on it (head, barrel etc), actually off RH75. Rear wheel is RM250 magnesium and that probably gives an idea of where Suzuki saved weight on the RH.
Back to the 20 kg question given that factory guys broke a few I would say the main area of weight saving is the frame, the TM was just a TS sans brackets. RH wheels were massively lighter, titanium bolts, alloy brake lever, lighter casings, alloy tank, it all adds up.
I mean no way was the TM an RH, just some bits can be interchanged, Firko's Cheney or building a lighter frame is the way forward with the TM. But Suzuki knew that, but for a factory that targets a 3 year model run at a production level I would say Suzuki's response was the RM ABC.