About chocks reducing effective holding power:
Warning: a light-weight pilot contribution
Doesn't it simply depend on the size and material of the said chocks?
If the wheels are so in touch with the chocks that they lean on them under power, then they transfer some of the aircraft weight to the tarmac via said chocks, and the contact area of the chocks as well as their coefficient of friction with respect to the tarmac will start to matter. If they are made of aluminium, for example, then the holding ability of the tire will probably suffer, as together they will slide along the asphalt / concrete.
Such chocks should still be somewhat useful when brakes fail and the wheel is free to roll, me thinks, so placing them a bit of a distance from the wheels would make sense.