Graeme, I completely agree with you. What seems to change the most for me, besides grain, with different agitation is the mid-tones. I use very dilute developer, and with variations in agitation, the highlights are always very well compensated for, but the midtones can really get a bump when I go from 5 min agitation intervals to 3 min.
I love to overexpose films like FP4+, Plus-X, or Fuji Neopan SS, and then do semistand with 5 min intervals. Sharp, very fine grain, and rich rich mid-tones. It works especially well in medium to high brightness range. Uniform lighting with little contrast - not so good.
I also agree with your sentiments on HC-110. I love HC-110, and use it a lot for rotary development, but for reduced agitation it'll be Rodinal all the way.
- Thomas