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why would you have lag? you would have hardly any with the amount of boost he wants to run.
My advice would be to get two junkyard t-25 turbos, and then send them in for a Big-28 conversion. This makes the turbos capable of putting out about 330rwhp per turbo, where as they normally make about 265 or so stock. After about 15 or 16 psi those small turbos are past their effeciency peak and just make hot air and less power. But since you only want to run 7psi no need for a T4 or such, they will give u slightly more power but noticeable more lag, not really worth it for a low bar pressure. You would have good throttle response and low lag with those modified t-28's, would be a nice setup and better then a Supercharger IMHO. On top end turbochargers are a lot more effecient and they rob the engine of less HP compared to a belt driven supercharger.
Those T-25 modified to "Big 28" would be much lighter then two T4's, also it would require a lot less space. Impalas don't have too much space for the headers surprisingly for the size of the car, would take some smart fabrication skills. Run two pipes forward into one intercooler that is setup for a 2into 1 configuration and run that back into your Throttle body with two Bov's close to the turbo compressors. I would also move your MAF sensor close to the throttle body and run it with a "blow through" effect. This is placing the MAF on the charded boosted pipes instead of the contempary style of putting it on the intake pipes. This would be simplier for a twin turbo setup, and allows you to run open atmosphere BOV's without stalling.(as long as they are before the MAF)
2 new custom 3" downpipes to stock cat location, remove stock cats, and use a Hooker catback system(has biggest piping diameter of aftermarket systems)
that would do it. If you want to "nig rig" the tuning, measure your stock MAF inside tube diameter. Take your current fuel injectors, for example lets say they are 100cc's/min. Now you run bigger power so maybe you want 200cc's/min injectors. This would be effectively doubling the fuel rate, so to fool the computer you will have to enlarge your MAF meter 100%, say the inside diameter was 2"'s, not you would have to cut out the guts and install it into a new pipe that is 4" inside diameter. Now the computer thinks it is just running the stock way, but you have put on bigger fuel injectors to handle the added power. You sneak the same amount of added air as added fuel past the MAF sensor and it all works. Technically this would work for any size injectors. But the stock ECU is for a naturally aspirated engine and it probably isn't maximized for forced induction performance(air fuel ratios)
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