Apologies for prolonging somewhat of a thread drift but I'M going to have to jump in here and agree with Richard.
The fact is that only around half of the surround roll contributes to the moving mass of the cone assembly and in the case of the drivers used by the Allisons, the size of the cone will form the majority of the mass in question. Hence the mass of the surround can vary to a surprisingly large degree before it has a noticeable effect. Also, it is true that in most drive units (certainly ones designed for use in sealed enclosures) the free air resonance and thus the overall parameters of the drive unit are governed by the suspension rather than the surround, thus demoting the importance of the surround parameters further.
Hence, whilst aware that a drive unit will generally sound different, I would say that re-foaming for a good but rotten driver is always worthwhile, particualrly as the effects will be less pronounced than replacing the whole driver with a modern 'equivalent' to the long-discontinued original.Equally, a re-foamed driver that sounds a bit different is still going to sound better than one with knackered foam!
The only other thing I would say is that I personally would NEVER re-foam a driver without removing the dust cap and shimming it. It can be done without, but it's far more hit and miss, espeically with drivers that have small magnet gaps.
Engineers: fixing problems you didn't know you had in ways you don't understand.