Also, I think your math is a bit off. My calculations show:
With 15% DW from gear (50% total), haste spell, dual 201 delay katanas, 23.73 gear haste you have 123.1527 delay (2.053 seconds).
With 15% DW from gear, haste spell, dual 201 delay katanas, 25% gear haste you have 120.6 delay (2.01 seconds).
Still comes out to about the same difference though.