Replacing the 747 with a JF-17 Block 2 changes things a bit: the fighter isn't detected straight away, with the first ping occurring 315km away from the site. However, even then, the track quality isn't good enough for a shot.
The S-400 has to wait until the Jeff is 240km away to fire. At this point, a 48N6DM is used to bring down the fighter.
Now, pulling out the big guns that the PAF doesn't have (yet), I'm replacing the JF-17 with a stealthy target (a F-35). The detection and engagement range falls to a low 79km. Adding a dedicated VHF radar, the Nebo-SVU, could makes things a bit better.
So, what we see is that even for conventionnal fighter-sized targets like the Jeff, the 400km mark isn't achievable. Even if the 40N6 was able to engage fighters with an high-enough Pk, the sheer size (and RCS) of the JF-17 makes it difficult to engage beyond 240km.
This could be made even more difficult by flying lower and using OECM. Now, if the S-400 cannot intercept fighters at 400km, what can it do? Short answer: a lot of things.
First of all, while it cannot provide air defense over the border unless located very close to the combat zones, it can nontheless provide a temendous amount of protection to Indian assets *in India*.
Located close to an airbase or an important C2 node, the S-400 can protect it from massive air attacks. If located close to the border, the system can act as a barrier to fend off air attacks by fighters too.
Perhaps the most important advantage that the SA-21 will give is to prevent the PAF a "free ride". By making flying high risky, the S-400 will force PAF planners to design more complex missions, with low flight, lighter payloads and increased wear and tear on men and hardware.
In its secondaty BMD role, the S-400 will also give India the capacity to deal with TBMs and SRBMs fired at the front line.
In the long run, it is probable that Russia will put into service a CEC capability for the S-400. Such a capability will likely involve the new A-100, the replacement for the aging A-50. If India can get its hands on some of these, then the 400km-range of the 40N6 will be usable
AWACS, transport and tankers would then be at risk even deep into the Pakistani airspace. However, this capability is likely still years away given how demanding CEC is.
Finally, the S-400 deal allowed India to secure technology transfers in key defense areas. High power radars, very long range missiles, and so on. For 5.5 billions USD, getting that tech and five S-400 systems seems like a killer deal to me.
Source: