Maybe you have not paid any attention to this thread since post #868 about MEGA SI that the NEW Helix 10 CHIRP MEGA SI GPS G2N models have that the current one does not have.
1200 kHz Side Imaging with CHIRP.
It is discussed in a few topics in the Normal threads:
http://www.bbcboards.net/showthread.php?t=763353
http://www.bbcboards.net/showthread.php?t=761616