Couple of things to keep in mind. Transducer and speed.
Transducer for SI in the Helix won't be near as good as the HD one that comes standard with the 999 or 1199 and not as good as the one that comes standard with the 899, wish I had done the upgrade and in hindsite I would without question.
Speed: With SI speed is critical, at 1.5mph you will not get good returns like you will at say 5 mph. You will be able to see some cover and structure at the slower speeds but not as good and the slower you get the less it will benefit.
Wonder if these units will accept the 999 transducer and is there a transducer swap program available for them????