8th (Coffee Lake) and 9th (Coffee Lake Refresh) generations are architecturally virtually identical and will perform the same at the same frequency and core count.
The only relevant distinction is the upgrade option on the 21.5" iMac where the i7 will have six cores with hyper-threading as it has an 8th gen chip instead of eight cores without hyper threading if it would have had 9th gen.
I have been asking this a bit, is the 8th get i5 8600 pretty much the same as the 9th gen 9600K aside of the clock speed? (3.1 to 3.7GHz)? Would that mean that a 8th gen 8600K (3.6GHz) and the 9th 9600K (3.7GHz) are pretty much the same?