hey didnt get to finish reading the thread yet (tight on time right now at work) but i got some pretty good experience to throw in! haha

anyways, my vote goes to an SV650 if you're really used to riding a twin. if you really really really wanna hit a rather steep learning curve get an i4 600. not saying its a bad thing, its actually quite a challenge figuring out how to effectively (keyword) ride a 600 i4...someone said it early, weak as hell until you wind them out, then they come on FIERCE. he wasnt kidding.

my background is learned how to ride a bike on a superhawk (not the smartest thing), moved up to an rc51 and began track riding nearly exclusively. rc51s are flippin HARD to ride, but the only thing that isnt intimidating/difficult to ride about it is that its really stable and the suspension is really good compared to a shawk. throttle and power is twitchy from the get go, it's ridiculously heavy, it takes some SERIOUS input to get the steering to work, even after steepening the geometry. but you learn alot from it. extremely rigid chassis that makes you understand how suspension and tires work. errors are very amplified, not much of a handicap available. you do get rewarded with some serious instant power getting laid down though. definitely change the gearing, it's geared way too tall to begin with. -1/+3 was awesome but you run out of room fast and makes modulating power very tricky. switching to a simply -1 made me go into too many corners too hot, which was a good thing, you learn that the limit is realllllly hard to reach on an rc51 haha. definitely get some rearsets, the stock rc51 pegs drag easily for some unknown and inexplicable reason. and let me make this crystal clear PARTS ARE BLOODY EXPENSIVE AND HARD TO FIND. make sure you get a spares package WITH the bike since the supply of replacements will likely dry out soon.

now, my NEW track bike is an 07 600rr, just got it a few weeks ago! rode it at pahrump back in september then when my buddy went overseas he asked me to buy it off him. amazing bike, really easy to ride, it basically makes you faster instantly. your mistakes are masked easily, the bike just shakes it off and keeps going. theres decent midrange for a 600. extremely light and flickable. power comes on pretty brutal above 10k rpm. you need to ride it that high to get any sort of drive whatsoever. most important point: it takes MUCH practice to understand how the power comes on and how to use it effectively. i've only done like 2 days on it and i feel like a newb, although a reasonably quick newb, but still a newb. tire feels like its going to spin itself silly and light up when you're driving hard out of a corner. doesnt feel like theres a gap between firing pulses to let the tire grip. its frickin tiny, the wind will blow you around. gotta get on the gas really early and probably a gear lower than usual.

i wanted a 600 to expand my learning anyways...i kinda hit a wall with the rc51 because its really hard to get over that glass ceiling. i still miss riding it when im on the 600 though, but the 600 handles and brakes so much better its not even funny.

hope this helps!