One of the issues with 'bounce' comes from where you support the keyboard, and what type of stand you use.

One thing I notice about the X50 is that the keyboard extends virtually to the end of the case, so support may be closer to the ends than a regular styled keyboard. Force applied past the support point is a major factor in 'bounce'.

I have two different types of stands. One is the X-type (I have several, but a discontinued Invisible is my favorite - weight stresses it to become more stable) and the other is the 'leg at each corner' type (Quik-Lok WS550) that eliminates bounce pretty much altogether on even the biggest and heaviest keyboards (my K2500 sits on it most of the time!). So what you use is a factor.

Your post DOES acknowledge the fact that your 30 lb.-ers do bounce. Do you play standing or sitting? I prefer to stand, most of the time, and this exacerbates the bounce issues by supporting the keyboard further in than an X-stand at lower settings.

And I have played an X50 (in a store), and that is a pretty light keyboard action there. Perhaps there is a tendency to not play it as forcefully as a beefier action?

Last point... and, BTW, this is an excellent post, addressing the ISSUE of bounce, rather than just stating a position about weight based on what we own .... To get the same 'bounce' issue as an arranger, you have to play them the same way - both hands on, often at extreme ends of the keyboard, often playing alternately, rather than simultaneously (multiplies the force moment applied to the fulcrum). Is it still not an issue played that way?

But for me, the issue isn't what weight bounces or not, I just have a definite preference for a certain key-bed (used it from G800 - G1000 - G70) which I find amazingly responsive and comfortable. Unfortunately, Roland chose to put it ONLY in some of their heavier keyboards (I'd LOVE a lighter arranger, but it MUST have the same key-bed). The A70 master keyboard is the only non-arranger they put this one into, and that weighs a fair bit, too! So before I get accused of PREFERRING heavier keyboards, the fact is I simply prefer this particular key-bed, and am prepared to play any keyboard, whatever the weight (under protest!) that provides it...