I recently upgraded from the 1500 to an 1800.
The biggest advantage in the upgrade to either of the new Yamahas is the addition of HDMI and the new HD audio codecs.
The first question that comes to mind is do you have a need for HDMI?
If not there is no reason, IMO, to upgrade. Yet.
Furthermore,
Reportedly, the new 1900 has the ability to pass HDMI thru the receiver without the need for the receiver being powered up. To me, should this be true, is huge. As this allows the ability to hear TV thru the TV's speakers at times when dynamic sound is not wanted. For example the 5AM news.
Sorry to side step your question but hopefully; something to consider.
The 1800 will drive speakers equally as well as the 1500. Not any better or worse. Not sure about the 863.