What about drilling small holes in the area where the wifi antenna is, creating a sort of a mesh? Could be very easily done with a drill with a small enough head by very carefully and slowly drilling into the aluminum.
I don't know if that would improve the wifi signal, but it is doable.
Might want to consider one thing though- People have been getting different wifi and GPS readings on unmodded primes, meaning that the problem is not necessarily the aluminum cover, but rather the quality of construction, components, etc.
The contact area of the pingions on the motherboard to the antenna are realy tiny,I think replacing the pingions on the mainboard for small coax will improve, The wifi chip is at the other end of the tablet, the signals have to cross the entire device. I even saw they route the wifi signal trough a series of PCB trace and cables to the place where they connect to the antenna. I think replacing these connections by new a cable with at one end an MMCX and the other end solderd to the antenna, will improve the reception.