I've actually found the TAR to be quite effective against Stealth Tanks. Just stay the hell clear of them, but 100 rounds can easily polish off a stealth tank and severely damage a flame tank. Nice weapon, and since sidney is only 150 creds, 3 or 4 of 'em can become unstoppable tank destroyers for a VERY marginal fee. I've also found that on C&C_Under, Sydney snipers can become a lethal tool. Catch the tanks as they go over the tiberium field and boom, let 'em have it. Most people will catch on, so you'll definitely need to keep behind something, but its definitely one of the most cost-effective tank removers in the game. Of course the rocket soldier delivers MUCH more bang for the buck in terms of vehicle squashing, he is mostly useless against infantry. Heh. I was driving around in a stolen mammoth tank once, three sidneys got locked onto my ass and the tank was gone about 10 seconds later. Hehe.
So my advice is: Never overlook Sydney, she packs a punch against tanks for only 150 creds.