One of Softimage's biggest drawbacks as far as its texture editing tools go is the inability to cut, sew and unfold UVs. It's something that's present both in Maya (which is extremely expensive) and Blender (which has not got full MSH export support right now).
Roadkill is an external app that deals with UV editing and includes these tools. This link also includes the xsiaddon plugin needed to get it working with your Softimage projects.
It works with Mod Tool and should also work with Softimage 2015.
You can download it here: https://www.moddb.com/groups/softimage-rs/downloads/roadkill-uv-pack-for-modtool-75