Im not sure I understand you correctly, but no charger will do that, to charge with 8v incoming to a 12v output, unless its a buck boost charger. Victron solar controller its not boosting only bucking, so it needs higher voltage to work. It starts charging at 5v above battery voltage, then it keeps on charging as long as incoming PV its 1v above battery voltage.I have used domestic panels now for over 7 years, firstly a 280w panel on my caravan, and then a 360w panel on my M/H. The only difference that I could see is that the domestic panels tend to be higher voltage, so use a good quality mppt controller that can take the higher input voltage. (280w panel vas 57v, the 360w panel is 36v)
The low light performance is a function of the controller, my victron controller will charge down to 8v input.
So at the light level that an 18v panel (Normal 12v panel) is at 8 v, my 32v panel is outputting 16v. So when the light is lower and say is outputting 10 v on my panel, I am still getting some charge whereas an 18v panel will be outputting 5v and not charging.
If you see any charge with 8v PV, thats not charging, its DC leakage to the PV.