Can I tell the difference between 400 and 100 in a 8x10 print, definitely!
I generally load up my M645 with 400 for general walk around, maybe handheld, maybe tripod, maybe monopod use. If I'm using APX-25 or PanF then it's onto the tripod we go!
I haven't experimented with how slow I can hold the 645. With 35mm I know I can easily go under the 1/fl 'rule' The other day I took a couple of pics, both handheld.. I took the 2nd without thinking that I was handholding (with tripod lying on the ground)
http://www.nlandgl.com/Film377/film377-01.jpg (1/250 @ f8 )
http://www.nlandgl.com/Film377/film377-03.jpg (1/30 @ f22)