First of all, there is a distinct difference between a 600 and a super sport. A 600, like a Suzuki Bandit 600 or Yamaha FZ6 can be fine for a learner, they are comparable to a SV650 in power and handling. A super sport, like a Suzuki GSX-R600 or Yamaha R6 is the kind of bike people tend to recommend against.
People will tell you that a super sport is too powerful, but that's not the full story. At normal riding RPM they have smooth power delivery and relatively little power available, meaning they are actually quite easy to manage. It's only when your RPM starts getting up that they start to fly. I would argue that if you have the self control to handle a SV650 (which are no joke despite being commonly recommended as a learner bike, they will still do 0-100 kph in under 4 seconds) then you should have the self control to manage a super sport.
The main disadvantage they have as a bike to learn on is the handling and riding position. They are designed to respond quickly to rider input, which isn't great if the rider inputs are the kind of poor riding a new rider is likely to do. The brakes are also strong and responsive, so they are more difficult for new riders to use smoothly. On to top of that the riding position is challenging, it is tilted forward over the bars and seems to promote putting weight on the bars, which is the last thing you want to do on these bikes because it ruins the handling.
As well as being more difficult to ride, they are more expensive to buy, insure and repair, and they are more complex to maintain. Having said that there are plenty of people that learn on them with no problem. Basically I would say they are far from the ideal learner bike for the reasons above, but they certainly aren't impossible to learn on.
Yes the Ninja 300 and SV650 are faster bikes than a 883. The 883 has about 50hp if I recall correctly, and weighs well over 220kg. The SV has around 70 hp and weighs around 190kg, so it will be miles faster, and the Ninja has around 35hp and weighs around 170kg. It has less power, but is lighter, handles better, and has a slicker gearbox, so it should be faster.
