Metter Chin upgrading your SSD won’t alter the limits of the systems CPU and firmware. While I do think you’ll gain performance adding a SSD it won’t technically allow you to upgrade to a higher macOS your system is limited too.
With that said there is an OS hack which will over-ride Apples limiter. But there won’t be much benefit as the features you are likely looking for won’t work in your system as the hardware that’s needed is missing. DOSDude Jumping up one OS release higher can make sense if your applications require it. Jumping two or more from that your system was limited to becomes more iffy!
Here’s the guides you’ll need to follow:
Swapping out your HDD for a SSD while easier offers less benefit than adding the SSD which is a bit more work. In either case any 2.5” SSD will work.
I think you’ll find this kit will get you everything you’ll need iMac Intel 21.5" and 27" (Mid 2011) SSD Upgrade Bundle to replace your HDD.
If you go with the dual drive setup then you’ll need iMac Intel 27" (Mid 2011) Dual Hard Drive Kit and the SSD 500 GB SSD or 1 TB SSD