You can't generalize about the ergos. Even among the lightweight class there's a lot of difference: the Honda is more stretched out; the KTM is very tall; the cockpits of the Kawasaki and the R3 are similar, but the Kaw has more leg room while the R3's saddle is a bit more likely to hold my butt in a single position. My advice: if you can't get a demo ride, sit on the bikes. Sit long enough to get a good feel for how it might feel when you're riding. If you can find a multi-line dealership, sit on as many as you can at the same visit. Just from sitting on the bikes, it was very clear to me that the Honda didn't fit me very well in comparison to the R3 or the N300, where I might've thought it was acceptable had it been the only bike I sat on.
I haven't even sat on a late-model 600, honestly, way too tall for me the last 15 years, but I can say that the '07 Gixxer 600 my husband used to ride had less leg room than my R3. Can't even really remember the cockpit. My '05 SV650 (which being a V-twin, was way underpowered compared to a 600cc inline four) was pretty uncomfortable, not for the space so much as for the shape of the saddle.
Motor wise - coming from a 600, especially a modern 600, it's not like you wouldn't notice the difference, but for me, the R3 has plenty. I'm still feeling it out a bit, but it's got more than enough to drop a gear and pass on a two-lane (and note that with an inline four 600cc, where the power band tends to be much narrower, I'd often have to drop TWO gears to pass). Plenty to get out of its own way on an on-ramp. Plenty torque to pick it up coming out of a corner. Plenty to keep up with your buddies, except for those times when they can't resist a long straightaway, and that gets old after you've done it once or twice anyway.
Where you really will notice a big difference is in the front brake. AFAIK all modern 600s have dual front discs, and IMO the R3 really ought to as well. I'll be putting on a steel braided front brake line in the next few days; others who've done that here say that that (possibly along with swapping for better brake pads) is enough.
Also, most modern 600s come with way better suspension. You can adjust conventional "non-adjustable" forks like the R3 has, by using springs with a different spring rate, adding spacers for preload, changing the weight of the fork oil and the height of the oil column - but doing it that way is obviously a whole lot more hassle than turning a click or two on adjustable forks.
HTH.
