Alright, I actually had a mod activated - the "Countries Modder".
I disabled it and for safe measure did the cleanup and repair...NOT throwing away my custom skins though. Then I did a very simple head-to-head mission at 2000m, Mig-15 vs Sabre.
The Sabre seemed to fly a bit quicker first but after a few turns it went back into flying at roughly 150kts. Again. Anyways, that might be how they want it to behave at low altitude. After this I changed the altitude to 10000m (30k+ feet) for us both and the Sabre chased me with more intensity. But still, even it's highest level of AI is a pushover if compared to the Mig's AI. It never even threatened me. I mean one must fly in a straight and level fashion for it to attack. Well, that's the case on my end. Might have to reinstall DCS altogether.
Or somebody else please try a simple dogfight with the Mig-15 vs an AI Sabre.