FlightPathOBN, you certainly have a good argument regarding the "compress" of the 2.5nm on the very final phase of flight.
In ICAO chapter 8, where reduced separation minima based on ATS surveillance is described, the minimum between two acft within 10nm of the runway end is 2.5nm.
This is surveillance (radar) separation.
But I would agree with Gonzo, 2.5nm is not a wake turbulence separation, or at least not described as such. I think the paragraph v) of the chapter mentioned above clearly states that 2.5nm may be used (apllied) when distance-based wake turbulence separation minima do not apply...
I also have a question for Gonzo; Who is speaking to the acft until 4DME, or where is the Transfer of Control point (normal operations, not low visibility) between TWR and the unit doing the approach phase? Do you "negotiate" about the visual final part for the every single acft in sequence and landing?