^ Well, I must admit that I am not that experienced with 2C-E, but I have taken it more than 5 times and this has pretty much been my experience. At 15mg, the effects were very benign for me. At 18mg it was a lot stronger, I would say at least twice as strong as at 15. I have taken it up to 22mg, which was very intense for me, several times more intense than 18. So yes, I would say the dose-response curve has been very steep for me.
I do believe that it evens out at some level (which is different for each individual) when the serotonin receptors start getting saturated, but that is probably at very high dose (comparable in intensity to perhaps 1mg of LSD). This is merely speculation on my part though, as I have yet to experience doses that high.
I'm not sure what to make of your case. Most people I know have had similar experience to mine. I suppose anomalies are to be expected in case of something as delicate as psychedelics?