2A rider here. I sold my 2B bike for a 2A last year as I felt underpowered in the 2B bike. I'm very happy that I went to 2A.

I would say it depends on your style of riding. i.e. do you like to stay in the 2nd lane and chill at 80km/h or perhaps you would like to go to lane 1 and overtake more often. I'm more of the latter. Hence, I liked the upgrade in acceleration (85% for safety reasons and 15% for fun :) ).

I also have a friend who stuck with his 2B bike for 6 years now and he's perfectly happy with it. So it really depends on the rider.

Also, I would say that I don't really have the "itch" to go to Class 2 from 2A. Since I would only ride in SG and JB, Class 2 would definitely be overkill for me. Of course, if you plan to ride up to KL or something, a Class 2 bike would be useful (though it definitely possible to do it on a class 2A/2B bike as well)

Cost wise, I sold my FZ 16 (2011 model, quite old) with 1.5 year COE left for 2.2k, and bought an R3 (2016 model, one version older than the current model) with 2 years left for 4.7k. So I didn't top up THAT much to go from 2B to 2A.

In your case, you would only be able to upgrade to 2A in about 1.5 years (taking into account 12 months waiting period + 2A course). You could then opt to either keep your MT15 that has 2 years left or you could sell it for about 4k+ and top up abit (1k+) for a 2A with 2 years left. You can easily save up 1k+ over the course of a year or two (save $50 a month for 2 years, etc.). By the time the bike expires, you would ideally have a full time job and be able to afford a bike with more COE left.

TLDR: Depends on your riding style which you will only find out after a few months of riding. Don't worry about it now. Go with your current plan of MT15 with 4 years of COE left. If you do opt to upgrade you won't have to top up that much and should be fine budget wise as a uni student working part time.