Mike
PhaseOne + backs are the clear leader as far as long exposures go.
I have gone 4 minutes with very acceptable results.
As mentioned above Dalsa chipped backs seem to run into trouble once exposures
pass 15-20 seconds.
Leaf has made very visible improvements with regards to long exposure processing with
the introduction of v11 and previously unuseable 30 second exposures reprocessed in
v11 were significantly improved.
Mamiya seems to hit the wall around 10-15 seconds based on secondhand reporting by
users on this site.
With the release of their new processing software,Phocus,later this year Hasselblad
is saying that exposures will increase to 60 seconds
Currently 30 second exposures using an H3D39 are very clean.
You referenced 30 seconds @ f8 as being a requirement for you.
From what I have seen,firsthand, that need is currently met by at least 3 systems
and I would suspect a fourth,Sinar,would not disappoint.
Mark