While it's true that you need to really know your bike and have a rather sensitive arse to notice the relatively small benefit you get from more aggressive timing or from an aftermarket exhaust.
However, whether you notice the difference or not you cannot benefit from the increased performance if you don't make the mod. You may not notice the extra 1.5-2.5 HP gain from a freer flowing pipe but it's there regardless. You may not notice the 2 lbs you shaved by opting for the titanium muffler but it's still lighter. A little here and a little there add up to a lot before long. I didn't really notice all that much difference in my old YZ after adding several mods over time. I got a complete exhaust system, V-Force intake, a high compression piston and gaskets, ported head, and cut up the air box. I did all these mods over about six months and when it was all done there was no step after which I really was shocked by a performance improvement. Then I rode my buddy's identical but bone stock YZ and thought something was wrong with it. His bike was much milder and seemed to sign off much earlier than mine. I didn't notice much until it was all added up and I compared it to a stock machine. I'll tell you what, a little adds up fast.