Sort of? If you can actually increase your performance architecturally by doubling your vector width then you double the power dissipated by the relevant bits

Sort of? If you can actually increase your performance architecturally by doubling your vector width then you double the power dissipated by the relevant bits of your execution and memory systems, true. But doubling performance by doubling power is really awesome. Things like your out of order window size will tend to increase power as the square of their size. And in your typical desktop cpu regime your power scales with cube of frequency. So if you're lucky enough to have code that can benefit form 256 bit vectors then AVX will double your performance and power then the thermal management system will throttle you back down to regular power and 80% of your regular clock speed for a net of merely a 60% increase in performance. Which is really nice if you happen to be doing all 256 bit wide vector math. If you're only infrequently using vectors your vector speedup will be smaller but so will the increase in power that needs to be overcome so it's still a net win.