I have read about people replacing their optical drive with an ssd or hdd, so is it possible to configure a two hdd raid 0 in addition to the ssd inside a 2010 27"?
Not very easily. You have to use 2.5" drives for the optical bay and the SSD mount, and the regular HDD is a 3.5". This wouldn't be a problem, except for it comes with a 1TB 7200 RPM HDD, while the only 1TB HDD on the market that will fit into the optical bay is 5400 RPM, giving really bad RAID performance.
Not very easily. You have to use 2.5" drives for the optical bay and the SSD mount, and the regular HDD is a 3.5". This wouldn't be a problem, except for it comes with a 1TB 7200 RPM HDD, while the only 1TB HDD on the market that will fit into the optical bay is 5400 RPM, giving really bad RAID performance.
Yep, not worth it. Replacing the stock HD with 3TB 7200rpm drive would most likely yield better performance gains than RAID anyway (unless it's SSD RAID).