I own a HHKB Pro 2 and a 87U. I just did a 55g swap to my HHKB and here are my subjective views on it so far.
55g is definitely snappier, heavier, and more tactile. I like it way better than 45g on the Alpha, but dislike it on the Mod keys, especially the spacebar.
My HHKB is also silenced and lube, which I think makes Topre 45g or 55g feel way better, plus it brings out the pure thock and removes any plastic-y clack (very apparent on my non-silenced 87u on metal plate compared to my silenced HHKB, they sound completely different.)
I actually found the spring on the spacebar to help with sound. Specifically on my HHKB, I had rattling without the spring and a cheap clack sound, I coiled the spring a little tighter and the tension helps especially grouped with hyperspheres and lube. Lube i used is Thick Krytox from TechKeys