However, Iran having the S-300 system also doesn't mean that they are now miraculously immune to attack. For one it would depend on how many systems they acquire (having a couple installations doesn't translate into a comprehensive integrated air defense network, and you can bet cash money that the Iranians have not bought enough to make up a S-300 IADs), their level of training (once again, it is doubtful whether there has been sufficient training), and such matters. And that is just looking at the Iranian side of the situation (the Israeli side obviously includes such matters like the Israelis not being utter fools, and thus having some sort of measures to use against the S-300). Thus Iran getting the S-300 doesn't offer them an aegis of protection.
With that said, the S-300 is not the Tor. The S-300 was not used by Syria. And the S-300 can give a lot of headaches if integrated into a comprehensive IADS (this is THE reason that the F-22 Raptor was practicing releasing JDAMs from high altitude at supersonic speed ...with its stealth and sheer kinematic performance the Raptor can eat S-300 sites for breakfast. The problem is that the same cannot be said for legacy fighters).
Anyways, I would be worried if Iran bought enough to truly augment their defense system. If they did, then the Israelis would probably be pushing for Raptor sales with even greater tenacity.