Well there is going to be a major change to this script soon as I'm going to be dealing with ImpEx2 soon that that will be using a separate MEMORY table that stores all the source/target id's (basically foreign keys with source import id).
This will allow for multi import 301 re-directs as there will be now the concept of a phase id, which is a new id for each whole form import. i.e. each merge import you do is a phase and has it's own ID. (p.s. yes this will allow for differential imports

).
There has been enough major boards and requests for this now. i.e. if one board assimilates 4 others with 4 merge imports, they will want 4 unique 301 handlers. And so they will be 4 phase id's to handle that.
Also to address the "Hey you only hard coded a few systems, mine isn't supported" ..... I'm going to try to add a drop in module system, much the same as impex itself that will allow for something along the lines of : "Ah, phase 2 is a phpBB3 -> vB import, I'll set the phase id in the 301 config file and drop in the module for 301 that is set for my urls". So then you can ensure that systems are supported. I'm guessing that the Tier 1 systems will be handled with logging so you'll get viability of the amount of SE redirects.