Yes there is an upper limit for depth of mulching, this is because the soil needs access to air.
6 inches is more than
enough to block out sunlight and keep all the moisture in the soil, deeper than that and you will create anaerobic conditions at the soil surface, this is not a good thing.
There are times when you can go deeper, usually this is done with pure wood chip that is fungal active already, you can grow lots of vegetables (not root vegetables) directly in these types of chips.
In England, I've read papers from two gardeners doing experiments on garden plots where they laid down 12 inches of decaying wood chips, planting most of their beans, cabbages, etc. directly into the wood chips, no added soil, and they produced quite well.
One other person has 6 inches of wood chips on her plot and the first year she separated the chips to plant, the second year she planted directly into her replenished to 6 inch depth plot and out produced her first year.
Her third year she again replenished to 6 inch depth and again out produced her second year.
The key to these systems great performance was that all were already decomposing through fungal activity.
Manures indeed release their nutrients faster into the soil and these products need to be either well aged (at least a year) or composted prior to use in a garden for best results with least possibility of pathogens being present.
There are mounds of research papers out there, just do a search on scientific sites that have agriculture, horticulture or biology sections (all will have at least two of these).
Redhawk