The state of tamim is only achieved when there is only one ruling principle in the soul, namely the tzelem elokim. Nothing else can affect the person who is tamim. Maimonides in his Guide to the Perplexed states that an uncircumcised person is more perfect physically. Since he is born that way he is more physically perfect. G-d created man uncircumcised, which must be a physically more perfect state respecting his physical existence.
Circumcision reduces man's instinctual drive.
It makes us less perfect physically but demonstrates that we must perfect ourselves spiritually. Milah signifies man's conquest over the instinctual part of his nature. Circumcision represents an institution in man which demonstrates a reduction of his instinctual drive. The instinctual part of man's nature is the source of his superstitious tendencies. Man's instinctual nature detracts from his being tamim.