I thought I'd post back here with the results of an experiment I did:
I got the 55g sheet in the mail a few days ago and swapped half of the domes in my HHKB.
I can't tell if the 55 domes are actually heavier. They DO feel FASTER.... and when I put on my headphones and loud music, I would say in a certain sense they have "more" tactility, I'm not sure I prefer it.
The "butteriness" of the stock HHKB domes isn't present, and it feel a bit more like my Cherry MX browns I'm typing on right now.
Perhaps most surprisingly, the sound COMPLETELY changed. On these 55 g domes, I now have a LOUD clack on the downstroke (upstroke already modded with hypersphere rings). I took of a couple of caps to see if it was the cap hitting the case or the slider hitting the PCB, and I think it is most likely the sound of the cap hitting the case.
I'm really stumped by the sound. It's present on every key I swapped. If the sound was similar, I might consider swapping the whole board - but it's a dealbreaker for me, and without the butter feeling of the topre keys I'm not sure I'd recommend buying these sheets.