Andy, can't help with your exact question, but many say that spirals work best when the dough size is within a specific range for that particular machine. According to Pleasant Hill

*Famag's IM-10 mixes and kneads up to 22 lbs. (10 kg) of dough at 60% hydration (or more at a higher hydration), but also handles batches as small as 3.3 lbs. (1.5 kg) equally as well. In terms of flour capacity, this is 8.8 lbs. (4 kg) down to 1.3 lbs (0.6 kg).*

I agree with you about the price differential, but went with the IM 5S due to size and wanting the option to handle smaller sized loads.