Which of the following exhaust brands adds the most power and sounds loud/deep tone as well? HKS hi-performance, BLITZ nur spec, APEX'i gt/n1, if there are any others that are out there that are better than these please fell free to suggest them.

I HIGHLY recommend the KO Racing TKO Exhaust system...

check out KOracing.net

how does the ko compare to the greddy dual exhaust?

I like the specs of the exhaust but damn...it sounds so stock, my brother has a magnaflow exhaust on his stock prelude that sounds louder than that.

On the KO Racing site they showed a test vs the Greddy and it had higher gains (but not significant, a couple horses and tq) but they have different Inch setups for different tones, I love the sound, you can hear all the exhausts for MR2's here...

http://www.madpsi.net/MR2ExhaustsPage.htm