Riding a bike fast, is all about controlling traction. Controlling traction is about being very smooth, limiting the transient loads on your tires, and understanding weight transfer, so that you know which tire has the most traction, and how you can take advantage of it.
so, the best bikes to learn on are softly sprung, with bad throttle response. The reason for that is that softly sprung bikes respond slowly, giving you time to think about whats going on, understand it, and adjust to it. Once you figure it out, it will become a reflexive action, and then you are ready to handle race replicas which have very sharp responce. They react instantly to what you do, and feel the slightest input. For example, a bikes handling changes sagnificantly based on weather or elbow is lying on the tank, or if its lifted up an inch. Now, if you dont have your body trained to do the right things (which are often counterintuative) than you are stuck with one of two scenarios. You can either take it real slow, not taking advantage of what your bike has to offer, and not being able to learn, because you are scared of your bike. Or you can go balls out, and crash your bike a few times while getting the gist of things, hopefully you will not have any permanent injuries.
The bad thing, is that learner bikes are not sexy. The sv650 and the 650r are more sesy than others, but are still lacking. The good news is that the unsexy bikes are cheap, and resale easily. Often you can sell them for what you bought them, after a year. Also, your insurance is highest when you start riding, and it really helps to have a bike that is cheap to insure untill your rates drop. So my advice is to pick up a starter bike, spend a season on it, then sell it and get a middleweight. By that time you should be running circles around someone who started out with a middleweight.
If you dont want to do that, go get a 600RR. out of the middleweights I've ridden (have not tried the 675) it is the easiest to go fast on, it feels the most neutral and predictable.
Have you taken your msf course yet?