I just installed a BBK CAI on my 08 GT, and so far have been satisfied with the results. Axel-backs are soon to come.

One upgrade that caught my eye are underdrive pulleys. Are the worth the money? Would there be a significant gain in power with this set-up? And is there a certain brand I should avoid?

 

They add a little bit of power. Not a whole lot, but it all adds up for sure. I like them and I am for them. I have been running Steeda underdrive pulleys for 5 years now.

 

Nothing like the gains the 5.0 used to get.. I bought them and hindsight says I should have saved that money and put it towards something better (like a dyno tune). You'll gain a little seat of the pants. The piggyback ones are the cheapest but you can get a misfire code if you don't clean everything perfect and torque the pulley bolts down exactly the same. You're better off getting a set that has the balancer included (non-piggyback) but that adds $100 or so. All the brands are pretty good - I tend to go with March.