It might also be worth cleaning the wheels with Isopropyl alcohol too. These new models were possibly given some sort of protective coating over the exposed metal parts to prevent tarnishing/oxidation. If such a coating has been applied, it is probably of a type of oil-based product. Just a thought....I could be totally wrong.
Earlier in this thread, comment was made about the Eureka 60 class and how its tractive effort can be improved. With my 3 Garratts, I went one (giant) step further and totally altered the method of boiler support; such that the total weight of the boiler now rests on the 2 motorised bodies instead of the inner bogies (which now just go along for the ride).
Unfortunately, nobody I know has come up with a method of utilising the weight of the front of the tender to add to the weight carried by the rear of the loco proper. I still think this could be feasible, but I'm stuffed if I can work out exactly how to achieve it (yet)......I've only been pondering this problem for about 40 years, so maybe in the next 40 years I might work it out. If I do, you blokes and sheilas will be the first to know!!! hahaha
I saw a similar method that you have mentioned, & I tried that & converted it back to how it was & simply put the extra bits of weight in the tank, along with the strip under the keeper plate. I have always had a 1:40 grade on my layout before the lead weight was added that grade provided problems, now it provides none.
While adding the flashing to the rear or bunker unit is not needed, I have a strip for it just in case.
One thing I have found with all my plastic RTR models is that as the are run in a bit, & the polished tyres get some dirt on them, along with the smooth polished tread also looses their lustre the models all tend to pull better. Good idea to run them around the layout for several sessions with a non problematic load & wear the model in a bit.