You are right, we could use a second version of the plane in the leightweight config with 18cyl. engines, but that's out of reach as of now.
Concerning the speed estimations, I take the liberty to put these back to still excessively high, but at least remotely believable and somewhat reasonable speeds.
780 kph... only in japanese dreams. The P-51H could go 780kph. Look at the two and you will notice that in order to get something of the Fugaku's design to speeds like that, you'd need to disarm and equip it with warp cores.
680 kph @12km altitude, that's what you'll get. That's still a lot more than one would expect from such a plane and it puts her into the same league like a Do 335 "Pfeil" (not judging "maximum" speeds claimed to have been reached in testing stage, but what the plane reached for real when it came to the front line).
Mike