I think it would be really difficult for me to hit 600 on my trainer if I weren't in erg mode. OTOH, I can keep my cadence up at 600 watts for only about 15 seconds and then I blow up. I did a couple of weeks of workouts like that last summer, thinking of it as being equivalent to weight training. Didn't seem to help me much. I enjoy it though. I have come to the conclusion that HIIT to do any good has to extend past 30 seconds/interval. Preferably a minute. If I wanted to increase leg strength, the best thing is probably to go at a wattage where you can recover from the trainer bogging down. So nowhere near 600 watts, at least in my case.