I've got to be honest, I'm unsure it's likely to be the resounding success they appear to think it may. It strikes me as a slightly-too-elaborate way of giving other lawyers an equity stake (it appears as though all their lawyers are primed to be able to buy shares at float) and, as you mention with DWF, the (albeit limited) track records show that law firms don't float phenomenally well.
Floats are also mega expensive; I get the whole "speculate to accumulate" thing but I can't help but wonder how sensible such a vast expenditure is in the current climate.
Plenty of firms break into other markets with strategic acquisitions etc, I think if a float is the way they plan to break into foreign (especially more insular) markets then they must be seriously confident that they understand the legal culture and landscape in those areas. Strokes me as much higher risk than strategic buyouts of respected local firms who have a genuine understanding of the culture.