I don't believe AMD has the resources to staff layout teams to update the layouts of all their different brands each time they do a stepping or process change. I think Turion shares the same layout - and thus the same transistors - as E-step Opteron and Athlon 64 parts, and that they achieve their lower TDP numbers by virtue of lower frequency and voltage, as well as leakage binning from the manufacturing lots.
Transistors are different, but who said anything about requiring extra layout staff? There are numerous tweaks to transistors that can be done automatically through various tools on a speed optimized design to produce a more power friendly design, all with minimal design effort. I think Intel uses this strategy on some of their parts as well. Start with 1 base design, and produce 2 silicon revisions from it, 1 optimized for speed, 1 for power. I think I read an Intel paper on this. If I can find it, I will post it.