Pretty good review...the one thing you don't mention (as far as I can see) is the real importance of running the head alignment. The Auto alignment does a better job of trying to do a manual alignment. I also note that you stated the manual said to print out with High Speed Off? You sure? It's been my experience that if you have properly aligned the heads, the print quality with High Speed on or off is the same. Obviously, if you have High Speed on, the printer is 2x the speed since it prints in both directions. To be accurate, this is my experience when working with a 79/9900 printer and not the 4900 which I don't have yet. So, this is something you should test on your 4900.
The other thing I would mention is that the head, with 360 nozzles/inch can handle high resolution image detail very, very well. With my P65+ back images, I routinely feed images through Lightroom at high PPI numbers up to 720PPI. Some images with high frequency texture may benefit from actually upsampling in Lightroom to 720PPI and then output sharpening (a huge benefit if printing from Lightroom as you don't need to spawn off different sized image iterations)...this is another thing to test for yourself.