I don't think Megatexture would be very useful here. The game looks to be using tiles for it's maps, which means lots of small, repeated textures. Same with the units.
Megatexture allows one to use a single massive texture in a game where the memory requirements would otherwise be prohibitive to use such a texture. I'm sure there is some sort of overhead attached to rendering textures that way. While it makes sense if it enables something that couldn't otherwise be done, it doesn't make sense to use it on an operation that would be inexpensive to begin with.
Regarding 64-bit, beyond the larger memory addresses, the other neat thing that "makes stuff go faster" is the availability of more processor registers. While registers are a very complex subject I'm still learning about, more registers means the processor can load more data into itself. For example it can load more instructions in one shot. My understanding is this is extremely nice in predictable situations like 3D rendering or video encoding, but I've heard mixed results for less predictable situations like games.
Furthermore, 64-bit isn't always faster in every case. Long division is known to be slower in 64-bit than 32-bit, due to having to work with larger numbers.
Overall, a lot of benchmarks I've seen show 64-bit performance has different characteristics than 32-bit, but is not always better in every case.
So I guess what I'm saying is don't get too worked up over it in any case