JT,
As Ivan sez, it is not "gross vs net", which are aircraft performance related terms. If you were designing SID compliant SDPs, then you would need to add your "net" decrement to the 3.3% or higher published gradient. The procedure design does not cater for OEI or TEI operations: the former is "any aircraft" and the latter is "aircraft specific".
The procedure design consideration of OIS + 0.8% reflects the environmental reality that preservation of an obstacle clearance height requires consideration of the exposure window. The further away the obstacle, the greater the opportunity for the elements to degrade the buffers.