Regarding your first question: When you change to low-resolution mode (8-bit), you do not affect the sample rate, but it can be used to improve the throughput back to the host computer. For example, when the digitizer is in 14-bit mode (and sampling at 100 MSamples/s), the device is storing 16-bit (2-byte) samples to device memory. Let's say you can fetch data from the device to the host computer (over the PCI bus) at 100 MBytes/s. That means you can fetch 50 MSamples/s from device memory to host memory. If you change to 8-bit mode, the sample rate (100 MSamples/s) is unchanged. However, the device stores 8-bit (1-byte) samples to device memory. The bus data rate is also unchanged, BUT you can fetch 100 MSamples/s from device memory to host memory (since each sample is smaller).
Regarding your second question: You can use a higher-bandwidth downconverter with the 5142. If it is not a NI downconverter, you need to program the 5142 with the NI-Scope driver (rather than NI-RFSA). There are examples included with the NI-Scope driver that illustrate how to program the IF features of the 5142 (Onboard Signal Processing examples. However, you should first consider the new NI-5663 Vector Signal Analyzer, which is 6.6 GHz device with 50 MHz instantaneous bandwith.
https://www.ni.com/en-us/support/model.pxie-5663.html
