It may be a little late but... could the cloning process have been interrupted before it finished ? If it had been complete you would have seen the exact same contents. You could open both devices in a hexadecimal editor like WinHex, then use the “Synchronize and compare” function, and scroll down, to see roughly how much data is identical between the two. If it's mostly identical through the end, then it's screwed, sorry, no way to recover data which has been completely overwritten. If only the begining is identical but then it's completely different, you may be able to recover many files from the Seagate Barracuda HDD, but by file carving method, meaning that you can't recover the original directory structure, the files names and timestamps and whatnot, since most of the metadata has probably been overwritten (I don't know the Macintosh file system though, but I guess it's similar to NTFS in that regard, in that most of the metadata structures are generally near the begining of a partition) ; it also means that fragmented files will be nearly impossible to recover completely, even if all their sectors are still there somewhere (
some programs claim to be able to recover fragmented files, it's worth trying, I have no idea how efficient that is). TestDisk wouldn't help in a case like this, but its companion program Photorec could find files based on their signatures (you should wisely select which file types are going to be looked for, to avoid false-positives). R-Studio in “Raw files” mode should also find something.
I've tried Stellar Pheonix once and have not been impressed. What it said about a 2TB HDD taking 100 days to be scanned is nonsense, it takes 5 hours at most on a healthy HDD. And making an image first wouldn't make the process any faster (unless it's copied to a SSD or a RAMDisk !...).