'''Q3:''' Hi! I have upgraded my iMac recently. First I had added the memory (4Gb->8Gb). iMac goes little bit faster, but not enough...((( Then I changed DVD with 240Gb SSD. And... I was delighted with increase in speed!!! It`is incredible! It really works!
'''Q1:''' You can! But u have to solve a little problem with a temperature sensor of HDD. Cause SSD hasn`t the sensor (Apple`s HDD has one!) u can`t plug the temperature connector to it. In consequence of this iMac speed up the fan to 100% - it`s noisy )))
'''Q2:''' Sure, u can boot!!!!
Notes:
I think it is not worth change HDD with SSD! Better way is replacement DVD with SSD and using SSD for system (OS X) and HDD for data. Or u can merge SSD with HDD in the one large logical volume by using Apple`s Fusion technology. I have done so. And now I have capacious (1T+240Gb) disk like HDD and rapidly one like SSD. I bought very chip usb-box for removed SuperDrive, as a result of this I have a nice external DVD.
It is not necessary to buy an optical drive kit. U need only special SATA cable (like in OWC Internal SSD DIY Kit for All Apple 27" iMac 2011 Models) and two pieces of the double sided scotch tape.