Just replaced the Teflon spacer after having similar issues, and having printed for several hundred hours. It seems that a lot of the problems that I thought were stemming from my high quality ColorFabb Standard White PLA/PHA filament (which didn't make sense) and poor prints can be attributed to the Teflon spacer. Below is a photo of what the spacer looked like when I pulled it out.
Prior to replacing the teflon spacer, my prints were stringy, and in general seemed to look worse and worse. In response I raised the temperatures to 225-230 - which seemed to work for a while. Then I noticed more stringiness, so I thought maybe it was the filament and lowered the speed - which also seemed to work, until it really no longer did. I have printed for hundreds of hours (500+) on my Ultimaker 2.
Hope this info helps!