Now you know why Ferrari chose more Softs than anybody else. They were much quicker than Williams on this tyre compared to the SS last year. Also, it looked to me like there is no more performance left to extract from the SS after pushing it for 10-12 laps. The performance kinda plateaus out after that, but the delta is about 1.5s/lap to the Soft during that window of 10-12 laps in the first stint (MAS vs BOT). Best pit lane time last year was ~29s. This time can't be made up by the performance offered by the SS in an extra stop.
Plotted Vettel's 1st stint for comparison in that graph. Didn't plot Alonso's final stint on the SS but the trend looked very similar to the first stints I posted earlier. About 10-12 laps of steeply declining lap times, then stabilising for the reminder of the stint.
It seems that the Softs can be pushed for much longer (~25 laps) than the SS before it too loses performance. IMO, the tyre strategy will be solely dependent on how racy you want to get and how much of a gap you have to your nearest competitor, in deciding to stop for new tyres as the SS reaches its plateau. It will depend if it's worth to keep going a bit longer for a consistent lap time to cover yourself for a reasonable final stint length.