Damage bonuses from Power Strike/WPF/Speed are multiplicative, as you get more PS, you benefit more from getting more WPF, and vice versa.
Raw damages using +3 Katana (40c), ignoring speed bonus and armor penalties:
45/3 15 PS 1 WM = 89.74
42/6 14PS 2 WM = 90.15
39/9 13 PS 3 WM = 89.4
36/12 12 PS 4 WM = 88.07
33/15 11 PS 5 WM = 86.2
30/18 10 PS 6 WM = 83.94
...So you can see, your old build did around the same damage, while 42/6 actually deals more.