What are the ups and downs of making the hitachi carb open both barrels?

Well, you'll notice more highway "power". The down side, might be gas mileage...that's what I heard.

I have done the mod myself on my old carb(currently I have a weber on the car), up side would be, when your flooring your subie, you will feel the different between when the secondary barrel is open or not. It is not much different but it still gives you more power than normal setup. As hti-johnson stated above, the downside would be the mileage, it will go down by depending on condition of your carb. At the time I did the mod, my carb was close to being thrown out of the car....and I got 14mpg...that is real bad mpg for subaru with stock everything(except for the bigger tires with peugeot wheels). i would said doing the hitachi modification is only temporary solution to gain little power. Go with weber, it will significantly increase the power.

Thanks

Dan