Old R6's are a different story. It's the newer ones that are fire bombs.
The newer ones (2008+) are also a significant improvement in nearly every aspect (suspension, brakes, power). The 08+ also received fly-by-wire throttle. I can attest to the performance of the bikes; with a ~100$ midpipe and an afternoon of work you have a full exhaust system. Add another 80$ and you can flash the Yamaha YEC race tune (Ie. change throttle mapping, replace spedo with thermometer, advance/retard ignition timing, change radiator threshold, remove rider safety limits etc etc.)
Of all the bikes out there, the R6 is probably the easiest to get race-ready from a performance/financial perspective. There's a good reason the engine hasn't changed much since 2006 and the "all new" R6 has the same engine. It consistently beat out other bikes in the 2006-2012 era and is still extremely competitive now.