This is an interesting thread to me, so apologies to OP for any hijacking
I guess the question I have is...where is the sweet spot? The answer, I suspect, is dependent on the equipment, and the target
My last trip out I got my alignment down to 10-12 arc seconds in both axes and I was getting 20 second subs at f/l 2000mm (C8) with round stars, 30 seconds I could see elongation. I'm sure it's diminishing returns without guiding, but I'm sure with a little more effort I could get longer exposures. I'll have to play with some darks to see just where my noise sweet spot is with my 1100D.