well ideally you can run lean if your not tuned. if you keep adding mods, your AFR will begin to go lean and sooner or later do damage. your car only adjusts fuel in closed loop fueling, outside that there is no vehicle induced adjustment. IE- open loop fueling
so if you add, intake, exhaust, DP,aftermarket BOV(shouldnt really make a difference), depending on what the factory AFR was calibrated to run at during WOT, its not hard to run it lean, especially if you add a boost controller to hold boost pressure at whatever u want it, instead of letting the ECU control it and drop it by time you get to redline.
IIRC the Speed6 drops boost from 16psi to like 9 or 10psi by 5000rpms. 6psi can make a big swing in AFR points if just a EBC or MBC is thrown in with out fattening up the top end with more fuel to compensate.
there is also an issue with the speeds with their advance tip in from cruise, i could be using the wrong term for it, but i have seen a lot of speeds pop motors going from cruiising trims to a quick WOT pull and bang. at cruise the Speed advances timing slightly to smooth out the economy side of things, and when the throttle is quickly pressed and boost is seen, it doesnt pull that timing fast enough and bam. again this is all speculation as well
