One thing that needs to be understood about "builds"... the radio is not part of the build. The radio is independent of the Android build. This is *WHY* you have a separate field in "about phone" for the radio and the build number. It is also why some *later builds* are being shipped with *earlier radio*. Most likely, the radio that ships with the "E" build either has some kind of compromise in it, or hasn't been adequately tested for distribution to everyone.
Also note that there is a distinction between the radio FIRMWARE, and the radio LIBRARIES. The radio libraries are part of the build. The radio firmware is separate from the build.
Whatever changes were made to the radio *libraries* for build E are most likely present in I and M as well. In fact, I and M could have radio libraries that are *even more* advanced than E.
For those saying that they get better signal on E than on D... this *could* be related to the radio firmware, or it also could be related to the radio LIBRARIES. It is also possible that use of the E firmware on D, I, or M could lead to inaccuracies in the values reported for the signal meter. In other words, it could *look* like a better signal, yet be exactly the same.
My first guess about the purpose of the different radio that ships with the "E" build, is that it has something to do with verizon's brand of volte. There was probably some weird glitch in the older radio build that demanded a quick fix.