Opera 12.15 makes better use from Gpu than opera 15. I used afterburner to see gpu usage during benchmarks. and compared.
For example, on Peacekeeper, one of the final test (crafty), Opera 12 was giving between 50-55fps, and opera 15 between 46-50 fps.
If you have a crossfire setup you will see that the gpu usage will vary between the two cards.
Gpu usage when doing normal browsing depends.
Normally hardware acceleration and webgl comes disabled by default in opera 12.
If you turn the settings to 1, it will occasionaly use gpu, but if you turn to 2 it will always use gpu,
And if you are watching videos on youtube with setting in 2; and if you open another tab and load a website you will notice that the youtube page will freeze momently because, gpu usage will be very high… I have 2 ati hd6950, and gpu usage when browsing, stays between 30-99% ( 99% happens more when you load a site, then when scrolling gpu usage stays between 30-70%).
Changing Renderer (preferred renderer) in opera 12 will influence browser experience.
I have found that in my system, using Opengl is smoother than directx, but with directx loads pages faster.
Opera 12 is stable on my Pc, the only problem i have found is related to facebook, scrolling is not smoother as Firefox Aurora, and opening some websites that have social plugins make it crash ocasionally , or increase load times, and that is solved with extensions.. Otherwise I can´t see diference in load times, that can make me change browser.