There are a lot of systematic problems with the network stack in the classic MacOS that I imagine will prevent you from getting anywhere close to wire-speed with a 100Mbit ethernet card. (The entire reason that Apple built UNIX-based file servers like the AWS95 and ANS 500/700 was that performance with the MacOS-based version of Appleshare server was *lousy*.) Granted, I don't even know where you might find good software to benchmark the Appletalk performance of a 68k system, so... how to confirm or deny that gut feeling is an exercise for the reader. (Find an ancient version of Helios LanTest, maybe?)

If you *have* a "NAS" of some sort set up (be it a fast Mac with a System 7/8 compatible version of Appleshare or a *nix machine running Netatalk) and can find appropriate benchmark software by all means I'd say try testing and see if you get anywhere close to "wire speed" even on the built-in 10mb Ethernet on most Quadra-class machines. I'd wager a nickel you get around half that.

(And regardless of the scores, if you're looking at attempting to stream uncompressed audio to a network share in real time all bets are probably off. For uncompressed "CD-Quality" audio you "only" need about 150Kbytes/second, so in theory you shouldn't even need a 100mbit Ethernet card, but I suspect the latency/overhead in the network stack will kill you. It's worth a try, I guess, but...)