hi mark,
the modeling light of the inons is rather useful to help position the strobes ... to support the autofokus they help only theoreticly ... because if you position the strobe straight at the subject (so the modeling light illuminates it) you risk strong backscatter (reflecting particles). in supermacro they are ok to support the autofokus, as you can straight aim the strobes on the nudibranch or whatever ... but with normal portait shots ... not so much. here a dedicatet focuslight helps more.
but: the focus of the sp350 will never be “fast” ... it might just get less slow.
good luck with the strobe.
serge