Surely SIMD combined with multiple streams would beat both approaches. (This would be separate streams in each SIMD lane and separate streams in different SIMD

Surely SIMD combined with multiple streams would beat both approaches. (This would be separate streams in each SIMD lane and separate streams in different SIMD variables.) There are multiple SIMD execution units, just like the 6 scalar units you mention. The latency of SIMD ops will be similar to scalar, except in cases you mention like shifts.