I made a similar upgrade in terms of price recently. The carbon bike is noticeably faster but also much more comfortable. Riding the same roads, with the same gear, in similar conditions with a similar effort level (as measured by HRM) I am about 0.7 - 0.8mph faster on average than with the Alu bike.
Now since I started cycling I've increased my average speed 6.5mph and I'm hoping for another 3mph eventually so that is a small increase in the grand scheme of things. But I always knew it was mostly about the rider's fitness anyway so I would have been satisfied to have any noticeable increase. The fact that it's so much more comfortable, faster and changes more smoothly means I am very satisfied with my purchase.
It's up to you whether you would see that as a significant performance improvement or not. Obviously just getting more miles in and the right training plan would be more beneficial, but it's even more beneficial to do both. Ultimately a lot depends on your own finances as to whether it's worth it or not.
As for Cube I can't offer any personal experience either but I can say pretty confidently that they are not rubbish. No new bike for anywhere near that kind of money is (unless defective). Some are just better than others and a lot of that comes down to personal opinion anyway. As long as you have a realistic expectation of the kind of performance increase you'll see, I'm sure you'll be happy with almost anything you buy at that price. I think the best thing to do is to try out a few and see what suits you best.
