There have been some discussions on the Leica zoom on Cloudynights:
http://www.cloudynights.com/item.php?item_id=2390
http://www.cloudynights.com/item.php?item_id=2795
And then this one comparing it to Baader orthos:
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
Re the Pentax zoom - I understand that the XF, while quite good, is inferior to its sibling, the Pentax 8-24mm SMC. I did have the latter for a while and it really was very sharp, had good contrast and produced very good views. I ended up selling it as I just wasn't using it enough.
Andrew