Even when Deoxys-S has 0 IVs in Speed, no EV investment, and a hindering Nature, it has 328 Speed. Therefore, your Greninja isn't as fast as Deoxys even at its worst. If Deoxys-S had 252 Speed EVs, a beneficial Nature, and 31 IVs, it has a monstrous total of 504, second only to Regieleki, which isn't in Gen 7.
Deoxys-S dips below 305 Speed when it has a hindering Nature, 0 IVs and no EV investment when it's at Level 93. Your Greninja will only outspeed if all that criteria is met.
Hope I helped!