Harveys' maps suggest that two of Arran's subbies (SubMarilyns) have in excess of 150m drop. In the case of Sail Chalmadale the drop is a suggestive 153m, and I felt this deserved some investigation on the ground. Hence a recent trip across the water.
Having crossed Beinn Bharrain, I descended to the col below Sail Chalmadale, which Harveys mark as 328m. I waited there for a couple of minutes, as close as I could reckon to the highest point, then noted a reading of 320m on my wrist altimeter (Suunto Vector). I trudged up to the summit, waited another few minutes, after which my altimeter read 480m, a difference of 160m from the col.
Now I am definitely not proposing this proves anything. I am aware of the problems with wrist altimeters and would guess that 10m is within their margin of error. However, I carried out the same procedure on the other suspect subbie - Beinn Tarsuinn - finding that the drop was reading 145m, quite close to the OS drop of 148m. It was a perfect day and there were no obvious reasons why the pressure should have changed in the few minutes between cols and summits. I conclude that there are at least a few metres of difference between the drops on the two hills.
In the absence of definite information from the OS, I doubt that it's possible to speculate further. However, the moral, surely, is clear. If you're visiting Arran, you'd be well advised to climb this one, just in case of future surprises.
