[slotloco]

"Firstly, if your rear track IS 2mm narrower, it is simply the setting of the wheels on the axles."

Dimensions taken from Slot.It specifications:
Merc 190E - Track 60mm front and rear.
Skyline - Track 58mm front and rear.

Looking at my example doesn't seem much scope for widening but will give it a go.

Specs can only get you so far. 
I cannot get any of them to 60mm, not even the Opel. You would have to have the tyres right below the body and rubbing when the body rocked in corners.
- I have about 9 or 10 units set up for racing, or as bench mules for scrutineers to use as benchmarks.
The pic below makes the Nissan look wider - but that's just an optical illusion from the camera lens angle, hand holding the phone.
The Nissan was at 58.7, the Merc at 58.5, but I could have shimmed the Merc just a poofteenth more.



Bottom line is as you already said, it is surprisingly competitive and stable.
Do you set yours up so the "sliders" touch the track and stabilise it in corners?