The other thing you do, by increasing contrast via agitation and compensating with shorter development, is lose shadow detail. The shadows get about the same amount of development from the same time in the soup, regardless of agitation, because the developer doesn't exhaust in shadow areas (thus bringing in fresh developer has little effect in those area). So, developing longer with less agitation to get the same overall contrast will increase shadow development (which increases the real film speed). Likewise, higher dilution (which also requires longer development to obtain the same contrast) will do the same, again because even the dilute developer doesn't exhaust in the shadows.
I've wondered if the main reason people find they need to expose at 2/3 to 1 stop under the film's rated speed isn't because they agitate too much or use too fast a process. I use a highly dilute developer, reduced agitation (10-15 seconds every 3 minutes), and develop for 3-4 times the manufacturer's times for "normal" strength developer -- and with the manufacturer's rated speed, I never have problems with lack of shadow detail unless I simply didn't give enough exposure.
Oh, and I get these effects with TMY -- in theory, they should be stronger with "traditional" grain films like Plus-X or Tri-X; that does seem the case with J&C Pro 100, with which I've gotten excellent shadow detail.