With ttcp I get about 2MB/s when the sender + receiver are
  on the same machine (a 25Mhz 486) & 285KB/s from this PC
  (with WE8013EPC) to a Sun3 (sunos3.5).

tried and got this.
				loopback	Sun 4/600 	Dec 5000/240
	486/100/NetBSD 1.0	2.5MB/sec 	650KB/s		950KB/s

the loopback speeds on the non-NetBSD were:

	Dec 5000/240 Ultrix 4.4	4.0MB/sec		
	Sun 4/600 Sunos 4.1.2	1.6MB/sec

Looks like with decent "other side" you can get close to 1MB/s off the
card but this box is never going to drive FDDI wire speed... (assuming
ttcp to localhost doesn't touch the NE2100 hardware)

-George