No it's not. You just budget your SMS as if the players will obtain bonus. If they do, perfect, if they don't, you may have some money for signing bonuses to sign guys at the end of the year for the following year. In reality there is probably some mathematical logic that gets applied to deals that allow GMs to forecast their bonus risks.
Sure, if you budget your SMS based on everyone achieving their bonuses... but when posters suggest "performance based bonuses", they generally aren't taking that into consideration, thinking that the base salary is what is being paid out.
Plus, alary plus performance bonuses are going to be substantially higher than guaranteed salaries, so if you are going to calculate your SMS with all bonuses being made, you are hamstringing yourself. Better to make the commitment to the lower amount...