Thats v strange mate ! what ROM you are using ? Stock ? , switch to a custom ROM and see what happens . I got the best battery backup from the S2 out of all the android phones i had.
Have you tried using BetterBatteryStats and look at its logs ? either you got a fishy app or you need to flash the stock ROM again using Odin . Or switch to a custom rom even !
And don`t worry i am quite sure S2 wil get the 4.3 . There is a huge dev following of S2 over at Xda