Tonight I was able to do another comparison. I hope I made it in a right way this time.
The aircraft I used was Aerobask Victory v1.1.3. The reason for this choice was, that I wanted to have a practical user perspective. I believe stock aircraft do not represent the majoritarian usage of X-Plane. This payware aircraft is not very performance consuming like others, but still a decent plugin based aircraft. The altitude of the Aircraft was 26000ft.
I've set the weather manually, any real weather plugin was switched off. One cloud layer was set between 10000ft and 12000ft, the 2nd cloud layer between 25000ft and 27000ft (all cumulus bkn).
SMP was set to max. cloud area coverage of 40000 sq. km. The fps is at ~43, quite good and the clouds are looking close to real clouds. Very beautiful. I have expected a higher density of the clouds, but like this it is ok as well. What I don't like is, you can still see the end of the cloud coverage area.
I have then switched off SMP with the plugin manager. X-Plane is loading default clouds then. If you now compare with X-Plane default clouds (tweaked with UWX textures), the second cloud layer I've set before has appeared. So the plane is flying in the middle of the cumulus clouds, as it should be. The fps are higher, almost 50, which corresponds to 14%.
As a second test I have increased the altitude of the second cloud layer between 31000ft and 33000ft.
But still no second cumulus cloud layer with SMP.
Back to default clouds, the second cloud layer appeared and also the end of the cloud coverage area is hardly or even not possible to catch. Also the density is higher and I think it is quite good corresponding to the weather datarefs of X-Plane.
What I have recognized regarding the missing 2nd cloudlayer with SMP, this will appear when the clouds are set to cumulus ocast. Maybe there is a bug.
So, if there is something wrong in my SMP setup, please let me know. I really eagerly want to use SMP, but my priority for clouds is not for the most beautiful shape, but more real appearance regarding density, different cloud layers and coverage area.