Maybe not much help to you, but I swapped the domes on my HHKB for BKE lights and although they felt great, they were noticeably heavier and louder than the stock 45g domes. I have some ultra lights on the way to try out, but from what I hear, even though they are lighter, they are also louder than stock.
Yeah I'm hesitant to mess with the HHKB, the sound is just
perfect to my ears (on a thick desk, it rings a bit too much on a thin desk)... and I have two SKCM blue keyboards.
I'm really just looking to try something else. Dome swapping a Realforce is a bit too pricey for my tastes, and I was not a fan of the stock 55g 87u, it just sounded too dead.
I do appreciate the advice on the weighting. It's only the SKCM blue that I find too heavy for continuous use, the 55g Realforce was fine, I hear the BKE Redux Light is fairly similar to 55g Realforce? I'd certainly look forward to what you think of BKE Redux Ultra Light compared to the 45g HHKB.