How much will adding an extra hard drive to my MacBook Pro affect battery performance?
Having a second hard drive will certainly decrease battery life by a noticeable amount. Behind the CPU and LCD, the hard drive is the largest drain on battery.

A word of caution: RAID 0 is almost never a good idea, unless you'll be doing backups constantly and don't mind being totally down if a drive fails. You'll get better performance from an SSD and you'll be able to keep your optical drive.