Oof, I focussed on this mod rather than your question about a guideline.
But in general I toss mods with less than +10 speed. Even for TM jumpers, speed isn't bad to have. It's just de-emphasized because of the kit.
I would do the same with arrows. Instead of making a hard/fast rule about which ones to toss, it's better to make a rule that follows a spectrum about which characters to put such a mod on.
Counter-attackers can take lower speed, but you can't count on them being hit, so I would happily put a +22 speed (or more) secondary / Offense primary on a counterattacker, but I'd be less happy with +12.
Extreme TM jumpers like Imperial Troopers under Veers can take the lowest speed you have, as long as you have one fast toon to get the TM train rolling.
Slightly less extreme TM jumpers that still have a ton of TM jumping (Resistance Trooper, Resistance Pilot, Geonosian Spy, Geo Soldier, Rebel Fighters under MM, etc.) drop those +12-+16 speed arrows on.
Assisters get the middle ground (+16 - +24), usually, because you get to control when they attack out of turn, rather than the enemy controlling that in the way that they might be able to do with counterattackers.
But yeah, read the kits. I **usually** (not always) throw out mods with less than +10 speed even if they've got a little bit of promise elsewhere, but you don't have to. With some kits speed is bad or at the very least needs to be somewhat reduced (think Droideka which gets bonus damage based on relative difference in speed between DDK and the target, also JKL if you use his lead).
But with other kits speed isn't bad, it's just not nearly as helpful as other things. So whatever is your minimum standard for mods, just keep the same minimum for arrows and make the question not about "How much speed can I lose?" but about "How much is speed prioritized or de-emphasized by aspects of the character's kit?" Find their place on the spectrum and assign non-speed arrows accordingly.