Hi again,
I started to do some research on these superchargers despite, what I believe, will be very limited interest from (UK) owners (even excluding the cost of parts).
Most of the kits I found were of 2015 ilk, or so.
Possibly of use to you, but maybe not with your much more recent power plant.
During a very quick search, none that I found, seem to offer alternate pistons to produce a lower (non-boosted) compression ratio.
I guess with present day electronics pre-ignition could be avoided.
We all know that Alphies and Vellfires are very popular in Japan, and there must be many 'customizers'. (Check Amayama).
I am sure HKS are a very competent and reputable producer; but what I did notice (in the HKS literature) is Toyota will not work on modified cars, and I expect after-market modification will invalidate all warranty claims; possibly important to you with such a recently made vehicle.
As I say, I have a factory supercharger fitted to one of my vehicles.
It transforms the car from a limousine to a racer by simple use of the right toe.
It also has Brembo brakes, very wide tyres, rear wheel drive, a limited slip differential, uprated suspension all round and is low to the ground.
It runs about 400 bhp and 400 ft/lbs stock. It is very quick when not in the usual casual limousine mode.
My Alphard has none of these features (but is top heavy when cornering), but still provides a very comfortable limousine experience.
So Good Luck with your quest.
Please let us all know how you satisfied you become, if you proceed.
Respectfully.