Yes the do not coast in N advice may well refer to the legality issue as it is technically "illegal" to freewheel, although in terms of back EMF there are no implications.
I have managed to see the percentage increase (only by 1%) using KERS down very long steep hills, reminiscent of descent mode in previous BMW autos, but watching the regeneration amps in this situation compared to the usage amps even on the flat, it's pretty marginal if significant on an overall charge cycle.
I'll be interested to see at what cycle life the cells begin to yield more capacity, having been heavily involved with rechargeable cells right through the NiCad, NiMh, Li-ion, Li-fe and Li-Po eras it could be getting on for 60-70 cycles to optimum which could be next year with my expected mileage!