Power consumption of the UART module on MSP430G2453

cmos current is pretty much always, the number of transistors * hz of transition changes.

The USCI is a simple module (only USI is simpler) so it have a low number transistors

So it's an tiny amount (I guess less than 500nA) while sending data and when not sending 'nothing'

Maybe not nothing, as there are always some leakage currents, but a few hundred pA will surely do.
During operation, the main current is external - driving the signal lines to the receiver. My guess is that this current is much higher than the internal consumption - and cannot be influenced by the MSP at all.

I wouldn't say the USCI is a simple module. With its many optional features (including IrDA, auto-baudrate detection etc.) it is quite complex (well, not as much as an USB controller, for sure). Especially if you count-in the I2C module. However, as long as those components are not active, they do not draw any current except leakage currents.
I never measured it, but I guess putting the USCI into reset if not needed, will draw even less than just letting it idle around. If really every nA counts.