Not seen any, certainly not independent testing, like I said: “what I’m led to believe”.
So … a bit of research, well Googling, came up with this piece - https://gearjunkie.com/oval-bike-chainring-review in which there’s links to a couple of papers. Unfortunately both link to the same one, the conclusion of that one shows no benefit - http://ro.ecu.edu.au/cgi/viewcontent.cgi?article=7377&context=ecuworks - or at least small benefits that vary between individuals. The other paper was meant to show an improvement but since the link is broken I’ve not, yet, found it.
The next result on Google was this - https://bikefitadviser.com/oval-chainrings/ where he assesses four studies. His takeaway about oval rings is, you’ve guessed it: it depends. He quotes parts of the conclusion of each study at the foot of the piece. Interesting the last has this:
We have interpreted our results cautiously and suggest that the subjects who had greater lower limb muscle volume and greater calf muscle volume, seem to have had a significant advantage in performing with the eccentric chainring.
Which sort of describes my rather large legs 