hmm, well, I suggest that instead of exporting from Blender as .xsi, export as .obj. A more universal format, but unfortunately you'll have to reset your texture data in Softimage.
Don't bother with 7.5, that's the official 'Pro' (meaning your $$$ on Autodesk's pocket) version. Get the 6.01 version, it's free.
Since you said that you tried to do the convert xsi, I assume that you've already D/Led the Forge Tools 3. You NEED Softimage XSI 6.01 Mod Tool to do the- Texture hook-up so that Sins will recognize it properly; hardpoints, like exhaust, weapons, and ability effects origins on the meshes, and possibly some other stuff.