There are afterburner effects, just not the ones you get on the player aircraft. If they are missing on your computer than there is something wrong in your dcs install, but it looks like they are there from your screenshots.
They are just much fainter than what you're looking for. Like Cobra said, they are the same fx on all the ai aircraft. I made a screenshot comparing the fx at night on an ai ajs-37 (i don't own the viggen yet) and the ai f-16. When i was writing this post i thought about it and went ahead and added an ai mig-21, which is an aircraft i do own and is activated.
As you can see, they are all the same effect and none of them are the fancy looking stuff you see in screenshots of player craft.
I don't think changing this is high priority for any of the devs right now. It might be an awful lot of work for something that isn't broken, but who knows.