What is the speed increase of using the largest buffer the OS will give you over that 1 GB buffer? Is that measurable?
I mean, you need at least an SSD, which write in blocks the size of megabytes. Many random reads will overflow their cache memory, because the individual blocks are too small.
But that's just the thing SSDs excel at: a very large amount of random writes. Because there is no mechanical movement. No need to wait for the head to slowly travel to the right location.
For the fastest possible, sustained sequential write of a large file, the main thing is to kill all other applications that read or write files, or even use the PCI bus. Kill all other running programs.
And at that point, it doesn't matter how large your buffer is, because the tiny overhead is washed away by having all available bandwidth for yourself. Operating systems are quite good at optimizing the available resources.