The difference in unsprung weight has more effect the more powerful the bike is (in the case of the rear) as it's primary benefit is to allow the suspension to react faster with less extension, therefore helping prevent highsides. Normally I'd say the little 620 doesn't have enough go to do much in the way of highsides, but I managed to highside a 675 last year so I'd have to agree, probably worth doing!
If things get really close this year I might invest in one, but I doubt I'll get to a situation where 2kg is going to make a difference to anything....