mii-diag output included the following:

Speed fixed at 100 mbps, full-duplex. #that's my linux box/card

Your link partner is generating 100baseTx link beat (no autonegotiation). #that's the switch

That (100baseTx rather than 100baseTx-FD) worried me too, but checking the mii-diag.c source, it's only distinguishing 100baseTx from 10baseTx in printing that message -- it'll never report ``100baseTx-FD'' in this message no matter what the hardware registers say. I don't know whether it's even possible to detect whether the far-end thinks it's in full vs half-duplex. Also, alas, I don't know what sort of errors you should expect to see from the Linux drivers if there is a duplex mismatch. It might be entertaining to force the Linux card into *half* duplex now and see whether throughput gets even worse!