I'm a believer in SS lines for steering precision. Perhaps it's that none of my boats with synthetic lines were ever pre-stretched/loaded, but in every synthetic lined boat owned, the steering was never up to the SS standards.
In my Stellars (SELs) and Epics V8, V10L, V10), I've changed out the gray, braided Dyneema for Q-Power Spectra. This was an easy retrofit, as Q-Power is rather stiff, making it easy to snake through hull tubes. It does not, however, hold a knot well; it's outer sheathing is almost too slippery. The difference in performance was noticeable, yet not drastic.
My Custom Kayaks Synergy Lite used a hybrid setup: SS until the last section to the pedals. It steered wonderfully. My Fenn double uses purely SS; again, no issues whatsoever. And nothing, nothing, steers with the precision of my Huki.
I expect that the synthetic lines have a higher tensile strength, perhaps even wear better, and are impervious to corrosion, but the 'wind up' under load is greater, and they have a greater propensity to slip at the pedals. Steel is real.