Hi Scott,
I'd recommend staying at ISO800, and dropping down to 4 or 5min shots. And do heaps of them 20 or 30. It doesn't seem to matter how long you go!! I know this sounds backwards & I've yet to get my head around it too. It's all about signal to noise ratio. With heaps of short shots the noise is going to be lower, so the signal will be relatively higher, and such you'll be able to "process" more detail. In saying that common sence still prevails. I don't think you'd get very good results from 500x10s shots!...
I think the exposure length minimum is dictated by the black point of the raw image. If you have to move the black point (in Photoshop for example) up alot then you have over-exposed. But if you don't need to move the black point at all then you have underexposed... I notice Scott that you aim for 33%-50% black point adjustment. I go for about 5-10%. Any more I would think is wasted exposure time. But I'm open to correction!
Does this make sense???? Perhaps I'm confusing even myself!!
The advantage with the filters you guys are using is that they essentially improve the black point of the raw image, so you can go a bit longer as an optimum exposure time. I know if I go for 20mins with no filter it's a complete waste of time as it's far too over-exposed. Mind you the detail is still there, but the blackpoint adjustment is crazy!
Phew.. This maybe all a load of rubbish!! It's just my mussings of the last few months.... I'm by no means an expert. Got a massive journey still ahead.
Cheers