Ah, I see now where you are getting the information, Mäx. I was looking at the description rather than the table on p. 163 SM.
In the table, it specifically states
Elemental effect (must be Physical spell with Physical damage) +2
Still, that makes me wonder if you can use the elemental effect (+2)
then apply the Stun modifier (-1)
kinda wonky, but seems in the realm of possibility. I also find it weird that some of the elemental effects (starting on p. 164 SM, sidebars) would specifically state they cause physical damage if there wasn't an option for them to cause stun damage.
Edit: while others clearly state that they cause Stun damage.