Regarding your cachefuly theory .. I completely agree .. I know that cachefly results are higher because of closer proximity, and I know the file hosting sites do throttle their input connections ... But having said this, you can still find a VPS which can overally provide better upload speeds ... And also, on my axisnext VPS, I'm seeing at if I upload to any filehost lets say 10 times, 6-7 of the times the speed is about 4-6 MB/sec, while in the other 3-4 times, the speed might be 11 - 14 MB/sec ... Meaning that the file host CAN accept such high rates .. So the fact that around 60 ~ 70% of the times when I get lower speeds, its because of sharing at the VPS end ... Which is why I'm asking for advice here ..

And just to add, I have seen file hosts accepting input data rates as high as 25 MB/sec, so your theory that they limit it well below 15 MB/sec is completely wrong ..