Avenger:
Well. as we fly all DME Arc in LNAV I would expect to see the appropriate RNP for the phase of flight, normally 0.3 for approach. While raw data monitoring is normal, no one in their right mind flys a commercial jet on a DME arc in HDG SEL..why would you? As for speed, VNAV will give a sensible speed base on the next limiting waypoint, as one contributor observed you would normally not fly a long arc at 180kts, in reality we could be 50 DME from touchdown.
A DME arc is an initial approach segment, so I would expect to see "terminal" or RNP 1.0.