To my unscientific mind a round coil should be more efficient than an oval coil. Equal distances from wick and juice. Less impeded air flow. Structurally stronger. ???
In my experience you are correct. Tried the oval coils because I wanted to wrap around 2 pieces of silica wick in a Evod coil so I would not need a flavour wick. Turned into a really hot vape because the in Oval form the coil does not keep it's shape well and airflow is negatively impacted. Add the fact that the top of the coil is much higher as well. So too hot to trot and way too much effort for no appreciable gain. Went back to round coils.