Any question answered...

Pretty sure I read somewhere (CTC mag?) that stainless steel isn't great for bolts - the high levels of chromium make them prone to fracturing. High-tensile steel bolts may develop some surface rust and look a bit shabby, but it's stronger in the long run.

You're right I think, stainless would be more brittle than 'weathering steel' or cromoly or whatever that still has some corrosion resistance. Thomson use grade 12.9 alloy steel, which I don't think is stainless, but I reckon that would be a pretty good bet if you were replacing a seatpost clamp bolt.