I have reverted back to the original theme, deleted all unnecessary apps (rarely use any anyway, apart from BBM), deleted all old messages, formatted my Micro SD card, yet it is still not performing well. But my sister who has had the same phone for the same amount of time, still easily lasts over 8 hours!?
The only thing that I think it could be is that I have ~500 email address on my phone, ~100 phone numbers, and about ~250 BBM contacts. Would this affect it though?