Hi Mikkel,
An interesting comparison, it got me thinking about why we might find some combinations look right and others don't. Starting from the prototype measurements, the ratio of sleeper length to gauge is about 1.91 for 9' sleepers and 1.81 for 8'6". Applying these to 16.5mm gauge, we get sleeper lengths of about 31.5mm and 29.8mm respectively. These sizes would give us track that is proportionally correct so hopefully would be more likely to 'look correct', even if it is not to scale.
Interestingly, Peco code 75 track has 29.5mm sleepers which are close, though fractionally undersize, to the 8'6" ratio. This surprised me as, before measuring them, I'd always imagined them to be much shorter in proportion to the gauge.
Whether all of this helps us understand what we percieve, I don't know, but it suggests that trimming the timber sleepers to around 31.5mm might give the best proportional appearance for 9' timbers.
Nick
ps. in my limited experience, staining timber sleepers with a wood dye or dilute acrylic wash before sticking the chairs down does not cause any problems.