Let me make a couple of suggestions:
1) If you get the headlight cutout extended up 2 inches (a good plastic shop charged me $20), you can have the acorn nut BELOW the rim of the speedo so there's no collision. Doing this also gives you a bit more screen down below which reduces buffeting.
2) I'm 5'10-ish and I see easily over top of a 19" with the headlight cutout extended up 2". Maybe you can do some more adjustment of your 15" even without cutting it so you can see over it? There's a surprising amount of ability to move the screen around on the brackets to lean it back and so on. Give it a try.
