Thanks for the purchase and for your question.
It didn't really come up in playtesting, mainly because I mostly presented stunts to playtesters as 'A little somethin' extra' on top of your successful hit, rather than very complex set-ups.
In the grenade round a corner example, I might rule something like the hostile around the corner is damaged but only takes 1d8 damage from the splash damage instead of the full 3d8. Or, if I was having a bad day, I might instead just say no damage was dealt and those are the consequences for trying something so tricky. What were your own ideas?
Rulings-on-the fly are very much part of Slipgate, coming as it does from OSR type games. Depending on your table, you may wish to roughly outline what the success or failure consequences could be before the player attempts the stunt, although you'll find that slows things down with players hemming and hawing before they commit. All depends on the sort of game you want.
The Stunt Die mechanic is shamelessly ripped off the Mighty Deeds rule for Warriors in the Dungeon Crawl Classics roleplaying game (though extra damage is explicity never a result of a Mighty Deed - slightly different) so if you wanted to read further about how people use that rule it might elucidate things a bit - there's certainly a lot more DCC players than SGCP players!
Let me know if you need anything else.