to answer your questions:
1. Has some one made that work or is the RAID controller restricted to only ports 0 and 1 (assuming optical drive is port number 2 as shown on the picture)?
for a g55, you need the mSATA and the primary drive for RAID. only these two have SATA 3.
2. Is there any performance penalty for the drive in the caddy?
i have g55 and i replaced the ODD with the second ssd. it works but as c_man stated it runs at much lower bandwidth. a more practical approach is to use HDD to replace ODD.
3. Is the optical drive also a SATA 3 interface?
from the speed test of ODD, i don't think so.
4. How about battery life - say playing a movie from the RAID?
i cannot access my mSATA (will void warranty), so don't have RAID set up. not sure about battery life.
maybe it's possible to boot from the ODD replaced hdd/ssd, but the speed is very much lower.