I know for a fact that a MiG-23MS will do in excess of 900 KCAS.
I'm not giving anything away when I say I can confirm that statement
(we would have especially have liked to have had the High Lark radar)
If you had, plus the IRST fit, your overall opinion of the '23 may have been of a different hue...
The '23 was a capable aircraft with robust and well-integrated systems. However, it was no '21 in a turning fight.
Anyone claiming to have flown both the '21 and the F4 and claiming they'd prefer to fight even in a late model F4 is either a liar or a fool. Or an overly-optimistic American.