I'll rank the ones that I've played (ascending difficulty):op. 2 no.1op. 8 no. 12op. 8 no. 3I can only span a 9th and I did not find these etudes too difficult to learn.
8/3 and 8/12 both have 10ths. I can't stand how they sound rolled though.
theres 11ths in the LH of 8/12 im sure you are away, but you roll these?8/3 is really cool, didnt look that hard from the sheets, what makes it difficult?