Not to sound like an ass, but why would not you go and read the wiki article, the link to which I have posted like three times already? Here it is again: http://en.wikipedia.org/wiki/HDV#HDV_1080pAs I understand it, there is no such thing as HDV 1080p (progressive scan)... only 720p.
They can speed it up, with corresponding loss in recording time. Roughly, 40% increase in tape speed would bring HDV to 35 Mbit/s, this is current XDCAM EX rate. This would lower recording time to about 40 minutes, the same as Digital8, not bad. With 80 min tapes this would yield the same one hour of recording. But I doubt they will do any of that, tape is winding down (sorry for the pun). Also, Sony would not want to hurt its XDCAM EX sales.The tape-based HDV is at its technological limit and probably can't be improved much more.