Oh, I shoot fast lenses like crazy (and love them like crazy)! FA31 is an F1.8, FA77 is an F1.8, and I have a 50mm F1.7 manual focus lens... and if I can adapt my digital, I have a DA*55 F1.4 waterproof lens.
The only thing with shooting at ISO 640 is that you lose some contrast? I really like shots that have that really 'deep' look to them. (full of color or tone) and I find that when I underexpose it becomes very air and light... almost kind of faded... especially in the shadows!