Grapevine has been: we do have 2 batteries of S-300 around Delhi...though hard to substantiate.
We never received any S-300 batteries. DRDO promised to deliver a system superior to the S-300 and hence was never bought.
S-500, if the program completes as intended, would be the real game-changer. The capability margin between S-500 (true ABM) & S-400 is much wider than that between S-400 & S-300, am I correct ?
Game changer, yes, but not as much for Russia when it comes to BMD.
S-300/400/500 are all true BMD systems.
S-300 can take out 2000-2500Km missiles, like the current Indian BMD can.
S-400 can engage 3000-3500Km missiles. S-400 just manages to reach the specifications set by the ABM treaty.
S-500 can take out missiles of the 5000 Km class. Of course, this is only official information and we don't really know yet. The US withdrew from the ABM treaty, so the Russians are leaving no holds barred with the S-500. So yes, the capability margin between S-400 and S-500 will be much wider.