After seeing all the benefits of using EXT4 as the filesystem of choice for external drives, I opted for it, and connected an external HDD to my Raspberry Pi, running XBMC.
I tend to get write speeds of 3-4MB/s for the first 2-3 seconds and then a steep decline in speed takes it down to around 300Kb/s.
Any thoughts as to why this could be happening?
UPDATE: I ended up giving up on finding the bottleneck. Here's the solution I employed. Partitioned the external drive to fat32 and exFat and by default I get nice high write speeds from memory card to external drive on both counts. This means that I can handle files >4GB too.