angus,
Well, that's different than just #63. The Autolite #63 are compatible with the XS650 and are the same overall length. Don't know about the Autolite AP63 Platinum, but I think I have a couple of them in my spark plug collection. I once had a list of about 20 different plugs that I tried in my faithful '78/E so I could recommend them or not based on experience.
All of these precious metal plugs have the same issue when used with a wasted spark system like the XS650 has. The idea is to allow the very small precious metal tip to heat up more than a standard tip because heat encourages the electrons to "jump" off of the tip toward the grounded electrode. A wasted spark coil produces a positive voltage on one of the plug wires and a negative on the other, so the advantage of a precious metal tip is only useful on one of the two plugs because only the negative wire actually has any electrons to emit. The positive wire plug emits the electrons from the ground electrode TO the tip. If you could figure out which wire was the negative wire, then you could save a bunch of money and only install the exotic plug in the cylinder with the negative plug wire!
If you are very brave, here is a way to do just that:
1. Remove the plug cap from one of the plug wires.
2. Hold a #10 pencil between the end of the wire and the spark plug.
3. Have someone else who is just as brave as you are start the engine.
4. Observe the pattern of the electrons as they go from the wire to the spark plug or from the spark plug to the wire.
5. The negative wire will show the pattern of electrons going from the wire to the spark plug as depicted in the above drawing.
6. Install the exotic metal plug in the negative wire cylinder.
7. Install a stock NGK BP7ES in the positive wire cylinder.
CAUTION: Do not do this procedure with the "Green Monster" coil.
The other way around this dilemma is to use a double Platinum plug, such as a Autolite APP3923 which has a Platinum plating on the ground electrode as well as a Platinum tip. Ford Motor goes one step further by installing a Platinum tipped plug in the negative wire cylinders and a Platinum plated ground electrode in the positive wire cylinders.
All of these exotic plugs do in fact produce better gas mileage and power, but in my experience using many of them, I found that the improved mileage and performance was at the expense of starting. The principle of their operation depends on them being super hot and when starting, obviously they are cold and do not perform as well as a standard plug that has a larger tip with more area for the electrons to gather.
