I could take a stab at it, but it's a "what if," and could be argued endlessly. The US test revealed a high speed on good fuel, but it was rated at about 438 mph on end-of-war Japanese fuel. I haven't really done much research on it since they only built 4.
We'd have to know the HP achieved for the US tests and the HP anticipated from the inlines, and the % decrease in drag anticipated. All of these would be a guess because the 483 mph I have seen quoted has been passed along over the years ... but I've never seen the flight test report that was cited.
Without that it is an exercise in frustration since everything is unknown.
So, the engines in it were rated at 2,070 HP. The DB inlines were mostly rated at from less up to about 1,700 Hp with MW50 injection at 13,000 feet. So it we take the Japanese quoted speed of 438 mph and assume a drag reduction of about ... say ... 15%, it would still come in at 434 mph ... or just about the same. But the 1,700 HP with MW50 was with German "good fuel," so it's hard to say. If you disagree, don't bash me ... post your own estimate instead.
But the Ki-83 could not have done anything but help the cause had they gone into production and had they actually been delivered in some numbers, at least from a performance standpoint. From a manufacturing standpoint, I can't begin to make a good estimate. Japan was hurting for manufacturing near the end of the war, both from running out or raw material and damage from continuous bombing. I have no idea how many Ki-83's might have been made or if they had pilots and fuel for them.
I suspect the Ki-83 might have been a bright spot in the otherwise dismal running down of their war-making capability. The individual Japanese soldier was always a good one. Their bravery was never questioned. But they did run out of good pilots and their products were having a hard time, quality-wise, near the end. Some of that was metal that was being made without all the needed components.
It's tough to meet production schedules when you have daily bombing raids on manufacturing facilities, few remaining good pilots, and little coming in as far as raw material goes.
WWII really WAS the first war to be decided by aircraft.
These planes were said to be remarkably maneuverable and are cited as being able to execute a 2,200 foot diameter loop at 400+ mph.