The only main difference is along the saturation axis from 0 to 100 according HSB readings or the Hue&Sat.-tool. As HSB is derived from RGB, one step (%) is more ‘Lab’-effective in a large working space compared to a small one.
You're right in that the saturation axis is the main issue, but you forget that Curves adjustments also affect saturation. In fact, the better I get at Curves the less need I have for the Hue/Saturation tool.
Here's some numbers to highlight the problem:
I just created a new image in sRGB (16-bit to minimize rounding) and filled it with an arbitrary colour ... in this instance 225,163,231. The Lab coordinates of this are 75,32,-25. Then I did a curve adjustment by placing a single random point (input 230, output 235) on just the red channel. This changed the colour to 75,34,-24. I then reverted back to the original colour, converted the image to ProPhoto RGB and applied exactly the same curve adjustment. The resultant colour is now 76,38,-23 ... as you'd agree (and expect) a much bigger change. In dE2000 terms, the difference has increased from 1.23 to 3.25. There's bound to be some rounding error in this but the fact is that adjustments in larger spaces just result in larger adjustments. Now you can just make smaller adjustments in larger spaces to compensate but if the adjustment you're trying to achieve is subtle, you may find this difficult. I wouldn't be surprised if a future version of Photoshop addressed exactly this issue.