Being a reasonably accomplished pro audio engineer I can confidently say that given appropriate recording techniques the memory size of the samples does matter. If you believe that Yamaha can make a sample with a few Megabytes that rivals one of several Gigabytes you are sadly mistaken. The reason manufacturers use lower amounts of ROM isn't because they can squeak phenomenal sounds out of a few MB of ROM, it's because it's cheaper for them to do so.
I welcome anyone to bring their Tyros I, II, or 3 (when they get one) over and sit it side by side my Wersi factory sounds or running a large sample VST library and see how well your keyboard fares. I already know which one is going to come out ahead and it won't be the Yamaha. I've owned the best Yamaha has to offer so it's not like I don't have a frame of reference to go by.
I'm curious whether you've actually played a Wersi OAS 7 instrument or a Mediastation? Given your statements I'd venture that you haven't. The Mediastation sounds far better than you and others think it does. Sure the styles on the Mediastation aren't similar to Yamaha's but sound wise the Mediastation is far better than you can imagine.
Wersi has an extremely well integrated and stable OS and I find it far more intuitive than Yamaha's. Even with the numerous features the Wersi has that the Yamaha's lack the Wersi is still a piece of cake to operate. Try one and see for yourself.