My other motorcycle is a Gixxer 1000 and I too was impressed with the Zero S torque and off-the-line acceleration. The Zero is a much better city bike than the Gixxer...light, maneuverable, and expecially fast off the line. And in this Texas summer heat, ICE bikes run very hot which can't be good for them and you feel the engine heat too, but the Zero is happy to just wait at lights or creep along in 95 F temps. Moreover, I don't know about your 600, but with my Gixxer below 11 mph, in 1st gear you need to be either accelerating or applying the clutch cause below that is too slow for the big bike.
The bigger battery would be a trade-off for sure. I too have the 8.5 battery. Longer range, but more weight is not a good trade-off for me. On my other bikes the choice was to trade weight for performance. A person really needs to determine their range needs before buying I think. For me, the range is more than adequate and the lighter weight is much better. Although I programmed the eco mode for the best possible range with lower torque and max regeneration and 60 mph max, I really don't use that mode. To me, the Zero is a performance bike more than a commuter or energy saver.