I want to confirm that it is NOT POSSIBLE to replace Intel Core i3 550 3,2GHz with i7 860 2,8GHz in iMac 27 Mid 2010. I was making upgrade for 2 27” iMacs Mid 2010, i got one i7-870 and i7-860 (this one by mistake). Both as replacement of base i3-550.
i7-870 works fine, Geekbench 4 scored it above 5000 in single-core and above 9000 in multi-core. Improvement of a system speed is noticeable, improvement of rendering is impressive.
The i7-860 processor scored about 1350 in SC and about 4300 in MC, this score is lower than original I3-550 which was about 2350 S-C and 5005 M-C. Graphic issues was also noticeable (some kind of throttling)
In spite of of originalmachead answer there is no problem with hardware, both i7 870 and 860 are the same year and technology. I think that there is no proper software for this processor in MacOS. Looking at Geekbench score - it seems that processor works 2 times slower because of: using 4 cores 4 threads not 8 threads.
Best regards