hi,
ive played around with imacs before taking the hackintosh route, simply because I was not going to for out 3.500 $ for a 5k iMac - so i built my own.. anyway back to your problem.
have started my journey with 24 inch imacs and run into this problem several times.
basically, as far as i know, its the SSD and compatibility with iMac hardware.
if you try different brand of SSD you might get full SATA 2 speed.
Ive googled this in the past, from memory its Sandforce based SSDs that dont achieve full speed on imac 24 inch.
so yea. pulling an imac into bits and to find this at the end simply sucks! but thats how it goes.
One other thing i wanted to mention, Apple quality control etc. its kinda ****ty. my last imac was 27 inch 2011 model and it only had a Western Digitial Black SATA 2 hard drive on a SATA 3 interfance.
where is the bloody quality control? i put an OCZ 256GB SSD into it and it went full SATA 3 - thats the imac that i sold and it funded 90% of the 4k Imac hackintosh i built