I'd say that it would be a stretch for the average 5.2 student to do MX1, although quite a few have the potential. I have quite a few 5.2 students and I'd say that some have the capability to try 3U. Often what happens with some reasonably talented students is they are used to cruising through maths in primary which causes them to become extremely complacent in Years 7 and 8 (hese are often the type of people who are good at maths, but need to really focus in order to ensure that they understand the work). As a result, they fall a bit behind knowledge wise which causes them to slip into 5.2. So it isnt rare to see talented students in 5.2. The main concern relates to the fact that MX1 has assumed knowledge exclusive to the 5.3 course. So a 5.2 student would need to catch up on the work by themselves, which can be an issue considering that a lack of motivation and/or skill is what landed them in 5.2 to begin with.
As for a 5.1 student, I'd say that it is a bad idea for them to try MX1. From what I've seen, 5.1 is usually reserved for the poorest maths students. As is the case for the vast majority of students, if you have the talent to do MX1, you should be able to scrape your way into 5.2 in the earlier years even with a poor effort. So I'd say that it would be a very rare case/set of circumstances where a 5.1 student has the capacity for MX1.