So is the option increase cost, or decrease attacks?
Keep in mind, a Space Marine, and most Eldar quad leaders, will easily have better then 5+ armour, so the AP isn't much on a problem. At most it can dissuade Ork player from charging, but even then they're likely to die from Str5 AP5 shooting. Weight of attacks can be an argument, but the quality doesn't sustain it.
On the Carbine Buckler, is the +1 attack too much? Would perhaps re-rolling failed armour saves of 1 be a better solution?