So basically OP is asking how fast you guys are able to transfer files over wifi and is there any overhead on using SSH.

I´m getting
up/down with ssh n900 and winscp (using scp protocol) 54 Mbit/s WPA2.
~450-500 kB/s (from/to rootfs or emmc)

.edit
python http server on n900 gives download speed of
~500 kB/s

..edit
Additionally Wifi encryption (e.g. wpa) could also slow transfer little. Still remember that in many cases you are able to connect n900 to pc as external memory with usb cable and it should be used whenever it´s possible if you need speed. (I carry this with me Micro-sub-short-cable(ebay))

...edit
6.75 MB/s is the maximum possible value. So factor in all the obstructuions and signal strength and you're looking at an average of maybe 2.5-3 MB/s actual speed on 802.11g 54 Mbit/s