I haven't flown in the premium class itself but I have flown on both TCX A330s (when they were JMC) and had a look at the premium cabin. You get bigger seat pitch than standard class, I think it's between 32" and 34" rather than 30" in Y class. The seats are also wider, arranged in 2-3-2 rather than 3-3-3. I seem to remember them being leather, although I don't know whether TCX have changed their interiors since they rebranded, so I can't guarantee this. You should find it fairly comfortable - I'm 6ft and the 30" pitch Y-class was just about OK
for me, so premium should be good. I don't know what complimentary extras you get, but if you check the TCX website you might find something on there. You get PTVs, but then so do Y class.
You also get priority check-in and priority boarding (unless like, when I flew with them, they only had one airbridge so JMC+ passengers had to get on last.)
Have fun in Cuba! I'm going there as well this summer, but with Monarch - whose seat pitch, I'm led to believe, is smaller
Come fly with me, let's fly, let's fly away...