We have about 23 people doing MX2 at my school. We all had to sit the first 2U assessment, in case someone would want to pull out - one girl did go back to MX1 so it was a good thing she sat the 2U assessment. This term only the bottom borderline MX2 students (about 4, I think) did the 2U as proper assessment. The rest of us still sat for it because our teacher thinks it's best to get as much exam practise as possible.
I would just study for the Ext-1 because it covers everything in the 2U. I tutor some 2U students in the holidays but a couple of them came to see me before the assessment and that was pretty much all the 2U revision I did. As for marks, last term was full marks, no surprises along with half my MX2 class. This term's assessment was just a few days ago but I feel confident.