[div32]

I often spot mic the cymbals much the way you do, and with great success. I usually place them about 18" above the cymbal, sometimes more or less depending on the sound of it and the drummer's style. I use XY with nice mics when the drummer and kit are really good (seldom). I spot with Octava mc012's.
With this technique you must really watch the phase. Check the 2 sm81's soloed and reverse the phase to see if you get a good centered kick and snare. Then add the kick and reverse the phase on it till you get the most low end and volume, then repeat with snare and other drums.
I also have to EQ the overhead tracks a little because often you get a slightly shifted image of the kick & snare, depending on the kit setup. I try to EQ to just get the sound of the cymbals.
Works great on thrashy aggressive music.