Right, managed to get some 3C509s to play nice with my Linux install (somehow can't do DHCP on anything that wasn't autodetected at boot - but with manal IP it works fine).
3Com 3C509-TP (1992 model) ISA 3Com 8350-01 6.06Mbps 1.0%
3Com 3C509B-TPO ISA 3Com 40-0130-002 8.41Mbps 4.6% (note: no duplex option in config, maybe old firmware)
3Com 3C509B-C ISA 3Com 40-0130-002 8.08Mbps 4.3% (note: duplex option present in config)
3Com 3C509B-C ISA 3Com 40-0130-004 8.60Mbps 4.6% (note: duplex option present in config)
Big difference between the -B and old non-B cards, with the -B being much faster, but the non-B seeming to load the CPU much less (even when taking lower speed into consideration), differences between the B cards seem within margin of error, it's all good stuff. Will now try some other ISA cards...
Edit:
These things are a pain, but starting to get results:
Artisoft AE2/C ISA NatSemi DP83902V 8.27Mbps 79.5% (NE2000-compatible)
Networth UTB16B ISA NatSemi DP83905AVQB 8.03Mbps 98.6% (NE2000-compatible)
Noname NE-12 ISA UMC9008F 8.09Mbps 95.9% (NE2000-compatible)
Well, looks like in case of ISA the 'noname' NE2000 cards are definitely NOT the ones to go for, at least not under Linux... managing to pull a P3-500E to its knees pushing through 8Mbps. Just wow.
Note that I was just using the regular NE2000 (module: ne) for the cards with that behaviour. Some offered alternatieve config (the Artisoft could use DMA), and other cards (3c503, SMC EtherEZ) offer shared memory, but I can't get those to run at all.