You're going to really struggle to find a bike in the same class that will be as good let alone outperform a FZS600. Much of the newer stuff is just not as capable, nearly all less comfortable, just ask those who jumped from FZS1000 to FZ1, all the Tracers are the same and forget carrying pillions in comfort, smaller, harder seat and higher foot pegs. Due to emission controls, the engines aren't as nice, often with snappy throttle response and higher fuel consumption real world. Yes some of that can be sorted but it's going to cost you.
Many of my customers have jumped off the FZS600 and wished they'd hadn't, some going back. The FZS600 was one of those bikes that was right straight off the drawing board and is in my opinion the best middleweight class bike ever made. It's not perfect but bloody close, does it all and very very well, out performing many 750s even some 1000's.
Good luck with your hunt, my advice would be to stick with the FZS
Unless you're looking for 1000 plus then go for a FZS1000 or XJR1300