In addition to a PowerBook G4 being ~10-50x faster (minimum) than a Quadra 700, Classic Mac OS is just slow at networking, and most software can't saturate the existing connections on these machines. Your PowerBook G4 will probably move faster on the network if you run OS X on it, for example.
Here's some benches somebody got a couple years ago, where they achieved a "noticeable" speed boost from ~3 to ~4 megabits/second.
I learned a lot from this thread, but it looks like nobody ever posted test results... I have some here for those still curious: Test file: Mac OS 9.2.1 Update.smi (84.4MB / 86,018,641 bytes) Switch: TrendNET TEG-S24g Gigabit Server: Power Macintosh 9600/1000(G4 [2MB Cache on board]) Apple...
68kmla.org
The whole thread is interesting, but the TL;DR is that this card doesn't do much in most normal circumstances. If I had to guess, it's "for" high frequency AWGS8150 and 9150 machines using software that matches things up with client software (like installing the network version of Speed Doubler on both sides) or "for" high-speed 8100s doing specialty work with NuBus cards that weren't replicated in PCI in time to move the work to an 8500 or 9500.
Tough to say though because even with a G3/400 the tests in the other thread barely broke 10 megabits under like one or two trials.
That said, the 10/100 card did go fast
er and I don't really know why.