I have looked at a lot of photographs over the past few days, and from what I can see the major issue definitely looks to be the gap between the bogie side frames and the running boards.
Looking at the photograph from Flickr by Don5617, of 4015, which is consistent with lots of other photos, the main thing that stands out to me is the distance between the brake cylinders and the running boards and the lifting point projections (located roughly above the centre wheel on each bogie).
On the actual locomotive, the brake cylinders are very close to the underside of the running boards, and the top of the cylinder is slightly above the bottom edge of the lifting point projections. On the model the brake cylinders are not anywhere close enough to the underside of the running boards. That alone is one of the main reference points that looks wrong.
However, it also appears that the brake cylinders on the model are mounted too low on the side frames, on the real thing it looks that roughly half or two thirds of the brake cylinders are above the top of the side frames. Looking at the real thing the brake cylinders look to be roughly in line with the cylindrical air tank, on the model they are somewhat lower.
It may even be that slightly raising the brake cylinders on the side frame may go some way towards lessening the perceived gap?
It's hard to tell from the photographs, but the gap between the bottom of the fuel tank and rails doesn't look too bad, and none of us are going to know for sure until the models arrive, but I think that the key measurement is going to be the distance from the top of the rails (track) to the bottom of the running boards. You would assume all dimensions from the running boards up are fairly accurate, so if the "body" is X-amount too high above the bogies to begin with, then the overall height will also be that much too high, which can be visually off putting against other locomotives and rolling stock.
Out of curiosity does anybody have a detailed drawing of the 40-class showing the actual height of the locomotive/height of the running boards above track level?4015 down goods at North Wollongong in 1966 (Don5617)
by Don5617 (flickr)