IME the extension is something I wouldn't do without. When I've gotten Macs to boot without the driver, I would get erratic behavior and/or nerfed performance. That's if they booted at all, I mean.
I know this might be hard to visually parse, because there's a load of other tests, but long and short of
this test is a native beige G3 300 scored higher than the test with G4/500 upgrade without the extension enabled—the "noExt" score. Now that very well could be the caches' being enabled, I'll admit. I don't know all the back-end workings going on.
However, the MacBench point
differential is almost the same above, as between my native 603ev 4400/200 and the 4400 with the L2 G3/400 below:
About 1,000 points difference, give or take.
The 4400 (and all LPX-40 derivatives) I've tried have needed the extension to reliably boot (usually one to two times and then they'll crash at system boot). The PowerWave and PowerTower I've tried with a G3 upgrade would technically work and boot each time, and I noticed marginal differences between it and the 604s that were in them originally, but with them enabled, performance was improved.
Now I'm not saying these tests are in any way conclusive, I'm just saying that these experiences are enough for
me to say, yeah, I'll always use the driver.