Be careful, you're confusing printer dots and image pixels. While some very expensive printers can overcome this, pretty well everything needs an array of printer dots to get the variation in ink density needed to represent an image pixel. Note that printer drivers can do smart things with the shape of that array. Anyway, 1200 dpi from the printer is somewhere around 150dpi in the image file. OK, maybe I have that last bit wrong. But I really doubt you'll see any better quality from increasing the pixel density above 300dpi, at least for colour work.