Yes, it really IS Britney.
The conspiracy thories are ridiculous. People hit out with this nonsense that she's been replaced with a look alike, but no one has attempted to put together a start to finish story of how/why she's disappeared and been replaced, because it would expose what a preposterous notion it is. No sane person would buy it as a full story.
Meth can alter a person's physical appearance quite significantly, and in a short period of time. Britney has denied using meth.