I'm not sure letting the compiler go wild would be such a great idea: one of the strengths of C is predictable performance, which would be hard to obtain if the compiler is allowed to e.g. move data across cache lines.
If you're running on one microarchitecture, I agree. But if you're running on more than one, manual structure packing may actually give more unpredictable performance than letting GCC handle it. At least GCC will make an effort to optimize for each one and hopefully avoid things that are absolutely terrible to do on that arch (orders-of-magnitude performance loss type stuff), while a manually chosen packing optimized for one microarch can be hugely pessimal on another one.
