MC309LL/A 2011 iMac SSD upgrade question

The Electra 3G will be noticeably slower. I don't know how obvious the difference between the 6G and extreme 6G is, but on paper the differences look minor.

The 2011 iMac supports three different SSD installations.

1. HDD replacement. This is the easiest, but requires a 2.5" to 3.5" adapter bracket and an inline thermal sensor cable as extra parts. You might get away with just tape, but those adapters/brackets are cheap so I'd suggest not cutting that corner. Some people skip the thermal sensor, but 3rd party fan control software is required if you do.
2. Optical drive replacement. Second easiest, but you loose the optical drive in the newest model to have one. A custom bracket may also be required.
3. Unused 3rd SATA connector. This one uses double sided tape because there isn't anywhere to securely attach the drive. It also requires significant disassembly.

Make sure you know where you intend to install the SSD upgrade, have the appropriate parts, and follow matching install instructions.

Edit: What SeaPapp said... looks like I was slow typing!

The OWC video you watched was to add an ssd to the system leaving the original hard drive in place. If you're going to replace the hard drive with the ssd then you should use a 3.5” to 2.5” adapter sled.

Going with OWC's kit is a good idea. Either ssd you choose will make a world of difference, make sure to use a 6G sata III ssd not 3G sata II.

The 2011 has a second SATA port, but it is buried on the back of the motherboard, and very very, very hard to get to.

You are far better off either using a Firewire 800 external enclosure, with power supply, or replacing the internal hard drive.

I buy my hard drives from NewEgg, the Samsung 860 Evo is known to work, the 870 is the newer version.

Make sure the seller is NewEgg...

https://www.newegg.com/samsung-860-evo-series-1tb/p/N82E16820147673

https://www.newegg.com/samsung-1tb-870-evo-series/p/N82E16820147793

You will need to install a thermal sensor, the old Apple drive had one built in...

https://eshop.macsales.com/item/OWC/DIDIMACHDD11/

This adapter tray will make life so much easier...

https://eshop.macsales.com/item/NewerTech/ADPTADRV/

You will need some tools...

https://eshop.macsales.com/item/NewerTech/TOOLKIT14/

When you get the screws off the screen, be careful!

Only lift the screen a little until you unhook the display cable. I slide a thin paperback book in to hold it up....

My usual upgrade path is to put the new SSD in an external enclosure, install a new copy of High Sierra, then migrate everything over. Now install the new hard drive...

Have you discussed both questions with OWC? They should be happy to help you with your OWC specific product and installation questions.