For the occasions where I've developed film in Dektol 1:2, I've always developed for two minutes. Only once did I developed a roll of Tri-X 35mm in warm Dektol as I was in too big of a hurry to wait for it to cool. I produced my best portrait from that batch of negatives. Very grainy and contrasty. I don't know about 8x10 whether the graininess would be all that obvious, but the contrast difference should be quite noticeable.