My only experience with spelt malt in beer brewing are a couple pounds I malted myself for a saison, so my experience comes from there. I've used spelt plenty of times in bread but it wasn't malted.
I found it very similar to wheat malt without much to differentiate it. I did not perceive it as darker, but perhaps weyermanns is kilned a little more. The dry malted grain tasted great; very much like white wheat malt tastes. The flavor was similar to wheat; perhaps there was a hint more "tang" but that may have been because it was home malted or the saison yeast. I did not pick up any character from it that would not be normally found in wheat malt; certainly no nuttiness of any kind.
As for percentage usage, I would say you could go up to 100% if you really wanted to. It should be fully diastatic with high power. For an american wheat I think you could simply swap out the wheat malt with spelt malt with no ill effect. It should perform the same and bring the same qualities to the table as wheat malt.
Just my opinion of course.