I recall a discussion with a photographer way back (20+ yrs) who maintained that using a 1.4x TC reduces or removes the donut shaped highlights. I can't understand why it would work, and haven't got a mirror lens to try it with, but maybe someone here can try it and give feedback. As for the effect in nicolai's example, that's about as good as it gets and even then I find it very distracting. In fact, it almost makes me motion sick looking at that pic. Sorry, just my humble take on it - no intention to offend.