I'm of the school of thought that is doesn't matter what the fly looks like (within reason), so long as it's the right size (or a little smaller) and presentation is perfect (or near perfect).
Size, shape, color, and movement in that order should be considered when selecting a fly for any water. I'm still amazed how midges can be so effective on big trout. When I first fished the San Juan River, NM, we threw #24 midges and caught rainbows and browns 20" and larger. The San Juan River is probably an exception, since it has such a huge biomass. The rule of thumb is that big fish prefer bigger flies, esp. browns. That is how they got to be big!