Well the smoke could just be any residue that was one the new mani and decat burning off, most do this when just fitted, however if it doesn't stop fairly soon then I'd start looking for a problem elsewhere. I'd advise getting a boost gauge first before doing anything else, so you know what the boost is running at, then also at the least an hks actuator to give you more control over the boost. The reason it may not feel as quick anymore is probably because you've had a slight boost drop and are maybe getting a little more lag with the free-er flowing exhaust, the hks actuator should help this, it'll definately make it feel quicker anyway.