I think I may have miscommunicated a bit...
I meant to say that the Gunther prop works great on a speed 400.
I did not mean to say a speed 400 works great in the Split.
I'm sure the plane would be quite fast with the S400 Zagi prop combo.
However, I think part of what makes this plane so fun is the large flight envelope. Keeping it light is what lets it slow down and and float around.
If you put a 400 in it, you will have to use a NiCd pack. The 720 mah NiMh pack recomended by the manufacturer will not supply enough current to power the S400. The BEC on your ESC will kick in.
You will be adding a lot of weight.
Like I said, I think it would be fine and actually scoot around pretty quick, but it's kinda fun to zip around the sky, then gently plop it down at your feet like a feather!
If you do use the S400 NiCd combo, make sure the carbon strips are secure on the wings!
Actually, consider HEcells to power your S400. They are NiMh cells that can deliver high current. You can get 1100 mah out of a cell about the size of a 600 mah NiCd.

Let us know how it goes!