No. The density is the same. The outside contains more blocks (sectors) but also on a much larger surface, so the density stays the same. You mean another effect.
No. Before the SSD-controller writes data to the flash storage, he must erase the appropriate data blocks, which means that the REAL SSD write speeds are always lower than the read speeds. Some SSD-benchmarks do not show the real difference between the read and write speeds, because they do use non-random data, which the SSD-controller can compress. Newer versions of the Blackmagic Disk Speed Test use random data, which is incompressible, and show the real write speed of your SSD.
From:
http://itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550?mt=12
Btw, uncompressed video looks like random data (in HexEdit for example), because it contains usually the complex details (from a HD-camera).
I was simplifying it to an incredible degree lol.
Also, for the second part, I was only talking about read speeds.