Even at a 1:2 ratio like I posted, you aren't getting super characters really, especially if you limit it to only points from one score can be applied to another score.
In other words, if you want that 18 (or 19 I guess), you'll have a 6 or 7 to trade for it.
11 - 4 = 7 (-2)
11 - 4 = 7 (-2)
11 - 4 = 7 (-2)
10 + 8 = 18 (+4)
10 + 8 = 18 (+4)
10 + 8 = 18 (+4)
Nets you +6 total modifiers, still no worse than on par with RAW currently.
Even a 2:3 ratio might work well enough, but for myself 1:1 would be too limiting and I'd just have 11's and 10's personally.