I'm assuming that you have other high-speed port adapters on bus 2, since you didn't mention the possibility of moving the PAs to bus 2. If that's possible, that would be the first move.
I don't know of any command that will show this, but your math above is correct.
The real problem here is that you're dropping packets. Just having high-speed port adapters in the chassis won't *necessarily* cause problems if you're not using them.
If you feel that you're seeing performance below expected, the best way to address this is through a TAC case.
Regards,
Clay