I think most domestic units have the facility to download the software and produce their own readings. I can tap into mine at any time and read the output and plot my own graphs of performance. the biggest problem is doing it scientifically - uv output can change with time of day, cloud cover etc. I've seen some fantastic claims on cleaners websites - probably bollox to be honest. I think I got about two percent increase in performance and that might have only lasted a few days depends on how fast the dust will fall again etc.
Domestic units can have the facility to monitor, but most people did not pay the extra money to do so when they had their panels fitted. Much of today's info at this seminar was aimed at frustrated PV installers who have have the guts dragged out of their industry. Now they are scratching their heads what they can do next. Experts there today were advising them that maintenance and monitoring is the way to ride out the current lull in the fitting side of things. They were advised to retro-fit monitoring systems to their previous customers and look at how to increase performance from the installed panels. A large part of which is cleaning.
Monitoring can take in to account the variables that you mentioned. Swansea uni spends tens on thousands on research, trying to squeeze an extra 0.5-1% efficiency from panels. I was asked by them to be a guest speaker today and did a PowerPoint presentation proving the benefits of spc with real life printouts, bar charts etc, from solar arrays we have cleaned. The professors there realised that cleaning has been severely underestimated and that's why they want to seriously pursue this avenue further. Some of the figures mentioned may sound outlandish, but if provable, you can't argue with them.
I agree that spc is very very circumstantial. Very few arrays are alike. Each will deteriorate at a different rate.