I thought the internal screening was done at 1200 ppi but sent to the pens at 300 or 600 dpi?
In any case there will be some interpolation happening to scale the 240 ppi to 300 then internally from 300 to 1200 for masking. You would not see much difference though other than in solid gradient fills, or perhaps diagonal lines depending on the frequency.
In my experience HP does an excellent job at scaling, I've not seen better yet (well Qimage of course) with other printer drivers.