The first difference mentioned is that whereas the first SSE2 implementations were often implemented using 64-bit ALUs internally, yielding roughly the same performance as doing two equivalent MMX ops manually, this isn't the case with AVX2. However, it may be worth noting, that it largely _is_ the case with the current AVX ("AVX1", i.e. pre-Haswell) implementations.
The second cited difference is that there's a 128-bit "boundary" in many of the operations. This is effectively what can throw down the drain the hopes of getting 2x gains over SSE2 just by naïvely migrating into AVX2. For instance, you cannot do shuffles to/from arbitrary components anymore, but have to consider the 128-bit lane boundaries instead.
The third issue, i.e. data layouts of internal formats and the assumptions of various algorithms are probably the most significant factors that determine how large a benefit you are going to get. Typically the internal data layouts (i.e. is my pixel block size 2x2, 4x4, 16x8 or something else?) are married with the ISA. Thus, when migrating from one instruction set to another, these typically may need to be reconsidered if speed is paramount. Interestingly enough, this means that when the ISA changes, you most likely want to do some higher-level algorithmic optimizations as well.
If you want to test without a physical Haswell, the Intel Software Development Emulator should work okay, albeit somewhat slowly. I'd post overall numbers for real Haswells, but Intel has apparently said we can't do that yet.
Regarding FMA, FMA3/4 are floating point only. Since x264 has just one floating point assembly function, only two FMA3/FMA4 instructions get used in all of x264 (not counting duplicates from different-architecture versions of the function). An FMA4 version has been included for a while; the new AVX2 version does include FMA3, but of course that won't run on AMD CPUs (yet).
XOP had some integer FMA instructions, but I generally didn't find them that useful (there's a few places I found they could be slotted in, though).
Note: I'm not trying to question your engineering chops, just trying to correct my own misconceptions.
sde -- ./myprogram myargs
instead of
./myprogram myargs
There's also probably a decent number of people at this point who have prerelease CPUs; they tend to breed quite explosively in the month or two before the official release.
