You will not notice FPS difference as accurately if you have a low value to begin with.
Moreover, some situations (like 25-man fights) are more both GPU- and CPU-intensive than others (like standing idle in a desert place).
And last, using an addon profiler with allow you to detected which addons are using the more CPU. But :
1) due to the way embedded libraries and the profiler work, some addons will get blamed for the libraries they embed, unless you ensure you are using only disembedded libraries,
2) this will give you the addon CPU usage amongst all user addons. It will not account for FrameXML (i.e. stock interface) usage nor all engine CPU usage (calculations, networking, ...) that can also lower your FPS. In the end, you cannot be sure that the addon that uses the most CPU is actually causing some FPS drop.
However, with these points in mind, you could try to replace your CPU-intensive addons with lighter one. It may or may not work.