As an Eldar player (the Tau are my second army), I would be most worried about the Hammerheads - especially if they were IonHeads with SMS. With three S7 and four S5 shots, they put Vypers and Warp Spiders (one of the best units in the Eldar codex - new or old) in serious danger. They also have a decent shot at knocking down a Wave Serpent or shaking a Falcon (thus negating its shooting for the next round). Shining Spears and Jetbikes look like they will be viable in the next codex, and the IonHead shines against them, too. The submuntion on a RailHead would be worrisome, but the solid shot is not that scary because it's only one shot - even if it's S10 (which gets downgraded to S8 against the Wave Serpent force field).
Sorry to disagree with you Mehphet'ran, but for XV8 configurations, I would go with the usual - Fireknife or Firestorm (depending on your preference) - rather than Deathrain. Bikes, Spears, and Spiders all have 3+ saves so a little plasma (or a high rate of fire with the burst cannon) can be a life saver. A good Eldar commander will not have his or her precious 4+ save troops (Banshees and/or Dragons) out in the open - they'll be inside the Falcons and Wave Serpents. It's true that Deathrains are good at shooting down those transports, but the Fireknifes and Firestorms are almost as good, and they can deal with the other troops as well.
One unit I would not bring againts Eldar are Stealths (sorry to disagree with you Spitfire) - the Eldar are just too mobile for them.
For the same reason I wouldn't recommend Broadsides, either. Even with ASS, they are not mobile enough. Moreover, two rail shots are not enough to really make a difference against all those Eldar skimmers. A third Hammerhead (especially if it's an IonHead) would be better in the Heavy slot.
I'm not sure that Kroot will be that useful, either. They get slaughtered by the Eldar melee specialists, and since the Eldar don't have much in the way of infiltration (except Rangers) I'm not sure their infiltration push-back skill is quite so necessary. Perhaps one unit of ten, but more than that is just giving the Eldar player points.
Fire Warriors in a Warfish, on the other hand, should be very useful. The new Eldar codex looks like it will emphasize Dire Avengers as the main troops choice, and Fire Warriors match up pretty evenly with them, for two points less a model. The Warfish is on par with the Wave Serpent, for about the same price, so you should be on equal footing in the troops area and you'll be almost as mobile as your Eldar opponent (but lacking the 24" move, unfortunately).
I hope this helps. Good luck!