I use an MFJ loop antenna on 40 through 15 meters in my garage attic "antenna farm" (restricted subdivision). I would estimate that its performance is about as good as an outdoor dipole. The tuning is fairly sharp, but is not a big deal for me as I operate mostly JT65 and JT9 these days and the sub-band for these modes is only about 4KHz. My loop is mounted vertically.
I made one JT65 contact from Ohio to Australia on 40 meters with both ends using an MFJ loop.
There are several videos on You Tube showing the MFJ loop working well on SSB.
I found them by just searching Magnetic Loop. I would post a link but my tablet doesnt let me.
The video shows them working well.
For my personal use I use a Pixel Loop for receive in the diversity setting on my Flex it's signal is lower then my other antennas as is its noise.
For what its worth, I can offer a couple considerations and a couple experiences I had in a similar situation.
For me, one of the draws to a Flex 6300 radio was being able to monitor a wide swath of frequencies at once. However, the very narrow bandwidth of high-Q transceiver magnetic loop transceiver antennas, to a large degree, seem to me to negate that benefit. Trying such transceiver antennas, I could literally see the signal drop-off on each side of the antenna-tuned frequency. And, with the relatively slow tuning speed of most of narrow-tuning transceiver antennas, its seemed that solution moved me even further backwards from some of the benefits of my FlexRadio.
I live in an extremely challenging, rf-noisy environment. Of the various whips, loaded, and end-fed antennas I tried, none could be described as being anything but complete failures.
However, I tried a Pixel Technologies (now called Inlogis I think) receive-only magnetic loop antenna. Though expensive, this type of antenna was successful in receiving signals where all other antennas were miserable failures (in all fairness, it's because I live in a concrete and steel set of buildings). And, though it is receive-only, it does not require any tuning. Thus, I can *receive* wide HF frequency swaths with no tuning or related delays.
Since I usually need to hear them to work them, in my case, receiving had priority in the all-antennas-are-a-compromise solution.
Your mileage and priorities may vary.
P.S. I have no pecuniary interest in either company. I got my Pixel RF-Pro-1B antenna from DX Engineering, but the other major ham retailers seem to carry it too.