If you only decrease the transmit power, your performance will decrease. The transmit power affects the coverage, the SINR gets lower and so on.
The 3GPP recommendations are given such that a ceratin performance can be achieved.
Based on the transmit power, there are different BS types: macro, micro, pico and femto. Our simulator supports either heterogeneous networks, or a homogeneous newtork choosing one of the BS types. Accordingly to the transmit power you have to reduce the inter-site distance between the BSs.