Another thought occurred to me.
You're playing a firing line arrangement against a light transport army. That's not a bad idea, especially considering how devestating a Pulse Rifle barrage is, but I think there is a better way. See, a Tau Fire Warrior line like that deals with an issue that an Imperial Guard Line deals with regularly. You're racing the clock on softening up his lines enough to absorb the shock of CC. What Tau have that Imperial Guardsmen don't have (aside from a better armor save) is a far more mobile army selection. You have Battlesuits!
Battlesuits can be equipped in such a robust way that they are the BANE of transports. They have mid-high strength weaponry at their disposal with moderate AP, and multiple shots. They have pretty nice armor saves. They have speed. They're expensive, but as long as you use that speed and don't engage the enemy you'll be alright. Take a small unit of Shas'vre Battlesuits with Plasma Rifle, Missile Pods and multi-targeters, and dedicate them to breaking transports. Once the transport is down don't even consider the contents, simply move on to the next transport. You won't devestate his numbers, but you'll slow him down while keeping your risk down. If an eilte unit or HQ has the speed or chance to corner them, they have high-powered weapons to try and assassinate a heavy points target.
Using your Fire Warriors, though not as many, take the time to pelt any transports that slip through your shock team of battlesuits. Any DE that get within firing range are dealing with a nasty barrage of Pulse Rifles. Play keep away and don't worry about getting into range for Rapid Fire. From what I've seen a DE opponent needs to close the distance in order to be devestating. Delay him by moving out of range and taking potshots.
Another two ideas for dealing with Transports: Skyray Missile Platform; Broadside Battlesuits.
1) Skyray - Ability to hit multiple targets with seeker missiles - Pretty much enough said
2) Broadside - I'd only recommend this if you're playing on a full 6x4 or larger playing surface, and only if you can tuck them away from the bulk of enemy with a clear line of fire. Those twin-linked railguns only need to hit to penetrate. They WRECK transports. A two battlesuit team is more than enough to take down any transport they'll have to throw at you, and it's not as if they're completely defenseless up close. If your opponent is using a bunch of cheap transports with stock infantry, stick with standard battlesuits. If they have an elite unit in a better fortified transport (or one that the general is running interference for) let your railguns rain sulfurous, supercharged, magnetized, mass-driven doom upon them.
Tau are built to handle transport armies by deploying specialized battlesuits. Even their stealth suits should be enough to give a DE general deploying transports concern. Even seeing the ridiculous amount of CC attacks DE can throw out, I can see ways of dealing with them.
My advice is nowhere near perfect, but I honestly believe Tau have a better chance of dealing with DE than most armies out there.