I think there'd be a lot of toons where you could do this. Especially, if the stats are good.
So your main question I think is this:
" ‘at what point is the speed sacrifice worth it? Anything less than -10? Less than -15?"
And for that, there's definitely no categorical answer. For 2 reasons:
1. Each character is different, and the loss of speed (for something else) is more or less vital.
2. Depending on how good the other stats on this mod are, how much speed are you willing to give up?
Characters fall into 3 categories:
1. Speed is vital.
2. Speed is very beneficial, but not vital.
3. Speed isn't necessary at all.
There's probably a 4th set (speed is detrimental), but that would be line up specific.
So category 1 & 3 are fairly easy to mod arrows for. It's category 2 that you need to make a case by case decision; depending on how vital/good other stats are on the mod compared to the speed you're losing.
Very very 'case by case'