Will the data throughput decrease if the transmitter and receiver are in motion?
I don't know why everyone else is focusing on the carrier frequency. The data throughput is a question about the modulating frequency (i.e., baud rate).

The Doppler effect applies to modulation, as well. If the transmitter is moving toward the receiver, the baud rate is increased by a factor of 1.000000092, and if it is moving away, the factor is 0.999999908 &mdash; the baud rate is reduced.

There is also relativistic time dilation, which is based on what fraction of the speed of light the relative velocity is. If it takes &Delta;t seconds to transmit some number of symbols, they will arrive at the receiver in &Delta;t' seconds:

$$\Delta t' = \frac{\Delta t}{\sqrt{1 - \frac{v^2}{c^2}}}$$

So yes, the data throughput from the receiver's point of view is reduced by a very tiny fraction at the speed you're talking about.

For a speed of 27.778 m/s, that fraction is about 4.28669&times;10-15, giving a ratio of 0.9999999999999957.