As with all of the weapons that Russia sells, they always hold something back from their 3rd world customers to make the buyer come calling again and again to update their weapons.
Just ask India about her experiences with Russia and all their complaints about buying or upgrading their aging fleet of Soviet block MIGs.
All of Russia's games and BS forced India to hire out the work to get better avionics, radar and weapons upgrades and repairs from Israel. The Russians are little more than cheap whores when it comes to selling questionable "quality weapons" of a technically advanced nature to desparate, ignorant and gullible buyers.
BTW, they did sell this system many versions short of their best to Iran, one of its very best customers.
Lets put the threat of the S-300 SA into perspective, even twenty plus years after Jimmy Carter's Betrayal of our great ally and friend the Shah of Iran in 1979, the ancient US made Hawk Missile batteries are still considered the gravest threat and danger to any potential air campaign against Iran.
Time is running out for Iran, several KC-135 refueling tankers are actually being serviced and "repaired" as we speak in Israel. They certainly won't wait until the election.