UK CAA AIC 17/1999 (Pink 188) para 2.5.1 refers to this issue.
It refers to the 757 specificaly in the approach phase alongside other types and states the seperation needed on approach is 4nm. No reference is made to it in the departure phase and we can therefore take it that standard medium v medium rules apply.
My company rules follow this official guidance and are perfectly clear. No additional seperation required on departure. As a commander I can choose to add an additional safety factor to any limit if I consider it necesary, that is an individual choice and supported by the company.
As this is a public forum and I do not know the origin of posters or their motive I'll politely refrain from revealing the company in question as we are not permitted to quote on its behalf publicaly. Sorry.