I run three Helix 7 in my larger boat. Bought a DI unit last year (non-Ethernet) and added two SI (ethernet) units this year. Also, picked up the MK TERROVA IP link, a network switch and a ton of adapter cables. I‘ve only had the set-up on the water once this year because the tow vehicle is down.
It looks like the SI will do anything that down imaging will do. Having two or more units networked allows sharing imaging between units. So for example, the bow mounted SI is connected to the DI Tranducer in the Terrova but you can also view SI imaging from the console mounted unit with a transom mounted SI transducer and visa versa.
The two newer Helix 7 can easily be moved to the 1448M Jon. I have other work to do on the Jon before adding electronics but the plan is a transom mounted DI transducer and a removable side mounted SI transducer on a carbon fiber tube. This would allow manual rotation of the SI transducer for a poor man’s 360 degree view.