From my understanding Stage 0 isn't YOUR factory stock tune, it's a stock tune that Cobb created. I'm not sure if they do the boost limiting in 1st and 2nd or not, but the map notes say
"COBB Tuning, v101.00 -- Stage0 v101 - Installed Stock Mode map for an otherwise stock 2014 Fiesta ST (B299ST). Uses stock values for conditions when you need the vehicle's ECU to act like it is still stock. DOES NOT UNINSTALL AP FROM THE ECU."
Since you can run 87 on the stock tune, I assume 87 would be ok with the Stage 0