OP:
Swapping out the HDD for a new SSD on the MacBook is easy. Do it right away.
It's much more of "a job" on the iMac.
Since you have a 2011, does it have thunderbolt?
You could add a thunderbolt-based SSD, and enjoy performance that is the near-equal of an internally-mounted drive, with a LOT less trouble.
TB-based enclosures are on the pricey side, however.
Might be worth it, considering the amount of work to get inside the iMac, and the chances that something else could "get broken" while in there...
Hey, I've installed the SSD on my Macbook Pro, and i'm happy with it. I'm considering using an external SSD via thunderbolt as you've suggested for my iMac.
How do I connect an SSD via thunderbolt externally though?