As for the range the Saab 105 uses 2 less powerful version of the J-85's and it has a furry range of over 2000km and you can compare this aircraft to other aircrafts powered by 2 none afterburner versions of the J-85's so 1,500km seems about right and if anything it seems to be a rounded down figure.
The ONLY figure so far that doesn't make sense to me is the 5.5 ton Empty weight! That's a bit off, I'd say that figure is more likely the standard loaded weight and NOT the empty weight. By comparison that's over twice the empty weight of the Jl-8(k-8)
As for the Airframe, the design it's self will allow for +Mach 0.9 max speeds and the engines themselves under the right conditions are more than capable of achieving those speeds however that's if the thrust to weight ratio of the aircraft allows it.
One of Iran's goals in producing this type of trainer was to have a subsonic aircraft to train pilots at higher G's at higher speeds with controlled high G maneuvers so it's possible that they overdid structural integrity on the 1st flying prototype however a lot of these specs will likely change if the aircraft is actually taken into serial production and the production cost would have to make sense for that to happen.