Hey Kastley, after million of search.. I found out why the drop at 2k-3k RPM..
its not about the stock exhaust, its about the Y pipes. and no its because its cheap..
actually all the Y pipes that have 2.5" and 3" merge will have high gains from mid to top but drop in low end...
here some examples..
Tanabe Y pipe VS MotorDyne Y pipe
Tanabe has 60MM dual to 70MM merge.
Stock Y pipe Vs Motordyne Y pipe (stock car)
so wow.. gratz on ur 40$ Y pipes.. cheap Y pipe ftw