The vmax topic is more complicated.
All sources point to that our version should do 593kph at SL and 740kph at 8400m.
But in what powers? This is not clear because I cant find the regime of ram at what the power of the sources are archived.
First we dont know if 594kph is at WEP or 100% since this is not specifed anywhere.
About 740kph at 8400m:
1. If military power 1660HP at 8400m is at 0 ram:
Then the vmax cant be at the same alt because with ram air crit alt will increase above 8400m, the vmax at military will be higher than 8400m and will be similar to what we have in game (around 10km).
So 740kph is not the speed achieved at 1660hp of military if this power is at zero ram, 740kph at 8400m will be the speed achieved at a higher power of WEP because a higher power decreases crit alt. With this logic it makes sense that 594kph is at WEP too.
This is similar to what we have in game:
In game data 0 ram:
100%: 1850hp at 2000m / 1660hp at 8400m
WEP: 2150hp at 2000m / 1980hp at 7400m
In game data at 740kph:
100%: 1660hp at 9800m
WEP: 1980hp at 8400m
Speed of J7W1 in game with this power:
100%: 564kph at SL / 739kph at 9800m
WEP: 594kph at SL / 756kph at 9000m
2. If military power 1660hp at 8400m is at vmax ram:
Then it would make sense that 740kph 8400m is the speed achieved at 1660hp at 8400m.
This would mean that overall the power is too high in game and with this logic is not clear if 594kph is at WEP or 100%.
If 1660hp at 8400m is at vmax ram we would get this powers (aprox):
0 ram:
100%: 1850hp at 500m / 1660hp at 7000m
WEP: 2150hp at 500m / 1980hp at 6000m
740kph:
100%: 1660hp at 8400m
WEP: 1980hp at 7000m
With this we would get speeds around:
If 594kph is at 100%:
100%: 594kph at SL with 1700hp / 740kph at 8400m with 1660hp
WEP: 630kph? at SL with 2030hp / 750kph at 7000m with 1980hp
If 594kph is at WEP:
100%: 570kph at SL with 1700hp / 740kph at 8400m with 1660hp
WEP: 594kph at SL with 2030hp / 750kph at 7000m with 1980hp
From my point of view I think the case 2 is the most realistic, with 594kph being the speed at 100%.
But there are a lot of asumptions like:
What is the power at crit alt of WEP?
What is the speed at crit alt of WEP?
What is the 0 ram power of WEP and 100%?
What is the speed at SL of WEP?
What is the power at SL of WEP with zero ram?
So taking in account the little info we have I think J7W1 is fine in game and could only be changed with new and better info. And if it is proven that case 2 is correct then it would be only be a buff for SL speed and it would be a big nerf to high alt performance.