Chris wilson did a detailed description about 6 months ago about why dump to atmosphere BOV's (ie the ones that go woooosh) should not be used on UK spec supras. Tried looking for it but cant find it. But yes, it makes the mixture lean. Basically, air gets measured and the corresponding amount of fuel therfore added at the injectors. However, between the measurement of the air and the fuel being added, the bov hasdumped some of the air to atmosphere causing the mixture in the combustion to be mad rich. The ecu then senses this and drastically cuts the injector cycle just as the bov has closed and huge amounts of air are coming back in to the cyclinders...hnce mega lean mixture for a small period of time until the ecu senses and compensates. Assuming you havnt backed off the throttle again and so the cycle repeats.
chris do you still have it ?
Jast as for the "seems slow bit" thats just because you getting used to it. Time for you to do some proper power mods now not these boy racer mods.
cheers
Neil.