You took several important variables out of the picture. Nothing wrong with that because from my experience, we do/did that all the time by either altering the physical structures of the targets or by 'handicapping' the radar via software if we cannot alter the physical structures.
If you sort of 'equalize' the fighters in every way, as highlighted below, then the burden of the kill falls
ENTIRELY upon the weapon. We have done this in the past when we are faced with physically dissimilar 'adversaries' but we must 'equalize' them somehow. The most commonly used technique is to install radar enhancer on the smaller body to where the estimated RCS is within 5% of its adversary's RCS. To 'hack' a radar's software involved too much time, possible security breaches and worst of all -- copyright related crap.
Anyway...If two fighters detect each other at the same time, and even if one shoot later than his opponent, assuming both fighters know full well the range capability of his missile, then it depends on missile's sophistication such as g-rating, the type of flight controls system, fuel formulation and shapes because they affect thrust and burn duration, missile guidance avionics...
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...In short, everything that we discuss about manned fighters, you can transfer to the missiles because a missile
IS an aircraft that have a higher performance envelope because it does not have to worry about keeping a human alive.
Here is the problem for your scenario...If one fighter is physically larger than the other, does that mean it has a larger RCS as well? Not necessarily. Even a B-52 can have an RCS of a bird, but at a very far distance. The problem is that if both fighters detect each other at the same time despite being physically dissimilar, it mean both have the same RCS --
FROM THEIR RADARS' PERSPECTIVES. An RCS value depends on the transmitting radar's signal and data processing sophistication. It mean a physically smaller body can have the same RCS as the larger body at the same distance because radar sophistication varies widely between manufacturers.
If two fighters of physically dissimilar sizes detect each other at the same time, then it mean the larger fighter have the superior radar system to compensate for its larger physical dimensions. If we assume that both fighters have the same radar sophistication -- no matter what -- then your scenario is impossible. The larger fighter will be detected first and will die first
REGARDLESS OF MISSILE SOPHISTICATION.
Let us use 1m2 at 100km distance for example. If both fighters that are physically dissimilar detect each other at the same time, it mean both fighters have the same RCS of 1m2 according to their respective radars' sophistication at 100 km distance. Get it?
It mean the smaller fighter have an inferior radar because if it have the same level of sophistication, it should have detected the physically larger fighter at 120 or even 150 km distance without itself being detected. In other words, assuming if both fighters have the same radar sophistication, the larger fighter would be 1m2 at 150 km distance while the smaller fighter would be 1m2 at 100 km distance. Who would die first? The larger fighter.
For your scenario that have an engagement between physically dissimilar fighters where both detect each other
AT THE SAME TIME the smaller fighter must have an inferior radar, and if both shoot at roughly the same time, then it depends entirely upon missile sophistication for the kill.
The reason why I often say '150-200' km distance for 1m2 RCS is precisely because of variations in radar sophistication. That 50 km distance variable is a terrible figure but it is the truth about the industry. That figure is about the distance for several missiles so you can see how important it is to gain even just 5 km of further out detection distance.
Is it possible to have even a rough RCS value guesstimate for any fighter? No.
But...You can place it in the same class -- base upon 'eyeballing' -- as long as you have a reasonably accurate RCS value from one or several aircrafts that set the standard for that class. The clean F-16 pretty much set the bar for 'stealth', meaning you must get below 1m2 at 150-200 km distance in order to be a credible 'stealthy' threat. So can you say 2.85m2 at 121.8 km distance based upon pure eyeballing? No.
Personally, I would place the JF into the F-16 class based upon what I personally know about the F-16's RCS and based upon my 'eyeballing' the JF.
I do not know how your friend had this 2.6 figure 'confirmed' to him. Absent assurance on how this figure came to be, such as if it was measured in isolation as in enclosed anechoic EM chamber, that decimal level of precision is dubious.
Same for the RCS reduction value of the DSI structure. Each DSI structure must be carefully custom tailored for the aircraft out of aerodynamic demands and because of that, its purported RCS reduction or RCS contributorship compare to the diverter plate is difficult to assess in regard to that percentage you cited. I mean...Were there measurements on the design that have the diverter plate assembly? If the design never intended to have the diverter plate in the first place, then how credible is that 30-35% figure?
That does not mean the DSI structure is not beneficial in trying to effect RCS contributorships from diverse structures on as complex a body like an aircraft. It is beneficial because you want to have as low a contributorship
FOR EACH structure as possible. On the other hand, if there is one or if there are several large contributorships from several different structures that utterly dominate measurements then it is pointless to debate on whether to install the diverter plate or the DSI structure.
To sum it up...It is reasonable to presume a 'class' but not reasonable to declare a value, and if said declaration involve a decimal point, time to be suspicious.