Sir, not to be rude, but I think you are asking for something out of reality's grips. I say this because as far as I'm aware, JB 4.1.2 is the latest software for the phone, Reborn ROM v2.2.4 is likely to be the most stable build, both consume a fair amount of battery life, to my observation virtually the same amount.... and asking for a lightweight ROM in the ICS or GB category is like asking for a Linux Build that doesn't involve terminal.. it just doesn't exist... However, you can look through other places like XDA developers website and see if they have anything.
BY THE WAY, I do recommend the Paranoid/Cold As Ice ROM, Reborn ROM v2.2.4, or possibly the CM10 JB ROM, however, the CM10 is relatively buggy for me, it does NOT have hotspot (have tried piddling with this phone for days) and it refuses to download picture messages. Also where did the MIC button go on the keyboard....
Anyway..... Try Reborn if you're cool on GB, it's totally unique though. Try CM10.1 if you want JB, and try Paranoid if you want ICS.
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Best regards..,.
Ben.
Maker of the 5400 mAh Optimus S battery.