with my current ROM JDmod.I9100.XWLPT ICS 4.0.4 and Siyah Kernel 3.5.2+ when i turn off the sencond Core then I experience a lot of lugging even when I change screens. I believe that it is acceptable for casual phone, sms and radio use.
On the other hand when i lower the CPU to 1GHz or 800MHz it is working very smoothly, almost the same as @1,2GHz. But I really dont see the diference on the battery usage. Only if I also lower the screen brightness to the lower level and close all notifications & auto-sync (facebook, tweeter, email etc) can last a whole day (with stock battery)
My conlusion is that for corporate usage, meaning Exchange Push email and a lot of calls during the day, the phone lasts for TOP 6-7 hours. The goal here is not the power save and deep sleep since i NEED exchange push email (That keeps wifi and CPU on), but to create a thin battery that can last more...
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Phone
Galaxy S2 GT-i9100 | JDmod.I9100.XWLPT | Siyah Kernel 3.5.2+