Neither Xbox or Gamecube hit their numbers 100%, but EVEN IF we assume GC hit their numbers 100% because of the fixed functions, it would still be less than Xbox if it was only capable of pushing 60% of those numbers.
That's simplifying it a little too much in a misleading way. Gamecube numbers are not more effective due to it being fixed function; if anything "fixed function T&L"
made it so that CPU vertex manipulation had to be used quite often, while you probably never used CPU for vertex/skinning/rigging on Xbox, having not one but two Vertex Shader units.
That's a bottleneck, that Nintendo compensated by using low latency RAM and supporting vertex compression transactions between the CPU and the GPU, to save bandwidth.
And shader model 1.1 wasn't all that programmable either; it just wasn't fixed function in the end, sure; but TEV was manipulable just the same.
And IIRC Xbox didn't do 8 texture layers per pass, those specs are wrong, lots of games did it via the polygon trick, but that's rendering the scene twice.
Anywho and getting back on topic: it's all about efficiency, the less hit doing something the better, but it wasn't strictly about fixed function.
Xbox was supremely bandwidth starved.
It was more a case of being riddled with bottlenecks.
GC's main RAM bandwidth was 2.6 GB/s, against Xbox 6.4 GB/s; memory controllers were 90% and 75% effective, respectively, which means GC could hit 2.34 GB/s throughput and Xbox 4.8 GB/s; that's still half the bandwidth, compensated by the embedded framebuffer, less main RAM latency (30 ns in Xbox and 10 ns on GC) and compression across CPU/GPU communications; embedded texture cache was meant to spam and ease out repetitive tetxures (1 MB of them) hence being used most often for EMBM, fur shading or ground textures; most of the time it gave "bottleneck free effects" (and repetitive texture spamming) more than easing out whatever (because had that not been there they wouldn't even attempt it just like they didn't elsewhere).
Xbox would still have the edge, if the latency wasn't so big and the thing wasn't so bottlenecked.
I don't really think the polygon performance of GameCube or XBox is relevant here, folks.
Indeed it isn't.