Aside from the placebo effect, it's tough to detect a lot of these mods, but my theory has been they'll all add up to something significant and help counter the natural deterioration of performance over time, and my experience--like the OP's--has been clearly positive in this respect. That said, there are some mods that scientifically guarantee improved performance (no matter how small)--like under-driven pulleys and lighter wheels--and others that are debatable as to whether they help or hurt--like CAI. I try to do the former and avoid the latter...