We have had the SEF coils for Minelab machines in the UK for quite some time and quite honestly your not missing much. I bought a 15x12 for my SE last year before the E Trac came out.
The SEF coils were getting Hyped up big time in the UK so I thought I would give it a go. They were claimed to be a lot deeper and more stable than the Pro coil.
I have regretted it ever since. The Upshot is they feel as if you have the weight of a sack of Potatoes hanging on the end of your stem compared to the beautifully light and balanced Pro coil.
The SEFs depth advantage for me has been maybe an inch over the Pro, no bid deal anyways. The Hypers claimed that it was way more stable than the Pro, thats rubbish, they are equally as stable. Minelab done a great job with that Pro coil, I am constantly dropping them hints at the factory in Ireland that we guys would love them to develop a 15-16" Ultra lightweight Pro coil similar to the standard one fitted to the ET. If they did that they would put a stop to these overweight, over priced, over hyped after market coils for good.
My pro coil stays on my ET permanently now. I go detecting for pleasure not punishment. Just my take on things.