Careful Attack/Strike is +2 to hit at the cost of your stat damage on the attack. I thought it was a mistake at first (because of DDXP) but then I started considering the math:
Let's say at first level I have 18 DEX (+4).
I can do my regular shot with +6 to hit, and do 1d10+4 damage (remember the bow has +2 prof bonus)
or
I can do Careful shot and have +8 to hit, and do 1d10 damage.
So I'm trading 4 damage for +10% to hit.
Fast forward to say... 21ish, and now I have about 26 Dex (+8).
Now I can do my normal shot with +10 vs AC and 2d10+8 damage
Or I can do my careful shot with +12 vs AC and 2d10 damage.
So I'm trading 8 damage for +10% to hit. I also hit twice as hard as I did before.
That's when it occurred to me that +to-hit is static. +2 to hit is always going to improve your chance of success by 10%, unless you are "off the d20" for your hit needs. 4E scaling is designed to keep you on the d20 when fighting appropriate encounters. So basically I'm getting +10% to hit for a scaling amount of reduced damage.
Let's say it stayed like DDXP (which appeared to give you Dex*2 vs AC for 1W damage.
At level 1 careful attack = +4 hit for 4 less damage
At level 21 careful attack = +8 hit for 8 less damage.
Why would you ever NOT use a +40% hit chance!?
Twin Shot is very strong, no argument. You'll burn through ammo at an alarming rate as an archer, of course.
But she can either fire 2 shots with +7 to hit, for up to 2d10+d6+2 (28, if both shots hit) or one shot with +9 to hit for up to 1d10+d6+1 (17) or a basic ranged attack with +7 to hit for up to 1d10+d6+6 (22). She's got to hit twice for Twin Shot to do up to 28, otherwise 17 is her max.
She usually used Twin Shot against minions or badly wounded enemies (so she could start on a new target with the second shot). Against high AC targets (like the dragon) she used Careful Shot, really needing to make it count. When she was down to 10 arrows, she started using basic ranged attacks.