I don't know about iMacs, but I know that a upgrading my late 2011 13" MacBook Pro's hard drive from the stock 5400 RPM 500 gigabyte hard drive to a 7200 RPM 750 gigabyte hybrid drive, and it wasn't a hack. I didn't have to use any extra software or any special Terminal commands. I just cloned my old drive onto the new one and that was it. I know for a fact that my MacBook's original hard drive didn't have any Apple firmware.
With a Solid State Drive, it's not firmware that you need to think about. With Solid State Drives, you have to deal with TRIM. OS X automatically enables TRIM for Apple Solid State Drives, but not for 3rd party Solid State Drives. For a third-party Solid State Drive, you'll need to use Trim Enabler, and you might need to disable SIP for Trim Enabler to work.