That's a tough question to answer and it's different for everyone.

I think Y+ is a good enough value that I just pay for it most of the time these days. I used to pay for business class with the lie-flat bed, but I've discovered that Y+ has enough space for me at a much lower price. I don't care at all about the meals, service or lounge access (because I'm a CX Gold, I get the lounges anyway). The main thing I care about is seat width, as I have broad-ish shoulders and I don't like sharing an arm rest with my neighbor. Seat recline is a distant second on my list and the CX PEY seat is fairly good at that too.

In other words, if I could purchase my seat space by the cubic centimeter, the CX PEY seat is about the amount of seat that I would want to pay for.

But I'm definitely in the minority here, I think the anecdotal evidence around Flyertalk suggests that most CX elites don't prefer to pay for Y+. Given the hassle of transferring your points and then maybe not getting upgraded, I would tend to think that making the upgrade attempt in your case wouldn't be worth it.

Regular economy on CX is actually pretty good compared to other airlines. And the K/M fares should allow you to choose your seats in advance so you don't get stuck with 2 middle seats in the last row. If I was in your situation I'd keep my miles, pre-select decent economy seats, and enjoy the flight. Just my $0.02.