Line speeds are measured in decimal bits per second. Data speeds are measured in binary bytes per second. The download speed you are getting is 300KB/s, not Kbps. Let's do the math:

2Mbps (line speed) = 2,000,000 bits per second

2,000,000 bits per second = 2,000,000/8 bytes per second = 250,000 bytes per second

250,000 bytes per second / 1024 bytes per KiB = 244KB/s (data rate)

The line has to carry control and address information as well as data. Typically, the peak true data rate is about 95% of the line rate, so you'd expect 244*.95 or 232KB/s to be the peak data rate for a line with a 2Mbps line rate.