Personally, I think that the 508's will be less comfortable than your 518's. They will come closer to you, they will have less width, and they will also turn your wrists out into the famed 'wheelbarrel' position. If you were shorter / had a smaller sleeve length, they might be ok for you, but I think that if you try the apes, you will be pleasantly surprised. A lot of that upper back / neck pain comes from the downward angle of your arms, which puts a lot of pressure on your shoulder / rotator cuff area. When riding, you won't notice it, but chances are you are holding onto the bars / pulling against them, and are not relaxed a lot of the time (especially in mountain riding wher you don't want to go off the road!). This will strain a lot of muscles back there, and also trap some of the gazillion nerves that run through the area.
As a test, when riding on a straight highway, throw on the cruise control, sit straight up, and start to elevate your hands upward as to simulate having apes... see if you experience the relief of pressure on that area. Once you get to about a simulated 10" of rise I think you will notice a HUGE difference.... 10" seems to be the beginning / magic number. Let me know!