I have had issues with the GPS date/time sync. It caused my clock to go wonky a few times and it set the time wrong.
You will definitely need a port splitter, otherwise RR will lock out the port you select for it and your navi software won't work. GPSGate is a good choice, I perfer it to Xport. Xport is free though, so that's a consideration.
The GPS speed volume control does not need access to the com port, that's based on IGuidance sending wav data and RR picks that up.