It is quite common to write "As you say..." in response to a written posting, though it is, I suppose, logically (rather than grammatically) incorrect. As there is a distinct time separation between written postings, it might be preferable to use "As you have said..." in this context. Constructs based on "As you write..." sound clumsy and artificial, perhaps because the time taken to compose a written reply suggests an alternative interpretation of this opening - which is along the lines of "something else you were doing at the same time as writing", for example, "As you write, you will be watching the sunset from your study...". Of course, this meaning is usually not consistent with the next part of the sentence, but it starts the reader off down the wrong track.
Another possibility which can avoid these difficulties and add variety to an exchange, is to use a construction like "As you pointed out...." or "As you correctly stated...".
Regarding the use of "yourself" in the question, it depends on the exact meaning you want to convey. "As you yourself said..." (or wrote, or whatever) is quite acceptable, but it has the specific effect of emphasising that the person addressed is the one who made the statement. It really concentrates attention on this, rather than the statement itself and usually will be followed by something which tends to use this statement to correct or refute another statement or conclusion by the same person. For example, "As you yourself pointed out, the far side of the moon is never visible from earth, so your assertion that an alien base there has been photographed from the Chile observatory, cannot be correct".
Hope that helps!