I have a G4/400-ed B&W, which was a straight swap over from a 400Mhz G3 module. Since the only thing that changed in the swap was the module I suppose that's as good as experience as any to comment on the difference.
Having the G4 module makes a "quite noticeable" difference in performance on certain mostly-unimportant things. For instance, the shrink/genie effect when minimizing windows to the dock is a *lot* smoother, as is the little icon-scale-on-mouseover effect. Having the G4 also vastly improved the performance of the "scan and scale" photo screensaver and significantly speeds up iTunes visualizations. But on more prosaic things like launching applications having it has positively *zero* effect.
(As for increased program compatibility, well, there's not much that runs on Tiger that *needs* a G4. Having it lets my B&W fire up Garageband, I guess, but it's still too slow to really use it. I wouldn't consider it much of a benefit.)
Was the improvement of those scaling/GUI functions more than a 25% clock speed bump would provide? Probably. Almost certainly. Just beware that if you use non-Altivec-aware programs the machine *will* actually slow down, as for "normal" instructions the G4's of the ZIF vintage don't seem to be *at all* faster than the G3s per-clock. So you might be trading, say, faster word processor page rendering for a faster GUI and iTunes.
Since you've already bought the G4 you might as well experiment and try it. Removing the firmware block won't prevent you from going back to the G3 if it better serves your needs.
Having the G4 module makes a "quite noticeable" difference in performance on certain mostly-unimportant things. For instance, the shrink/genie effect when minimizing windows to the dock is a *lot* smoother, as is the little icon-scale-on-mouseover effect. Having the G4 also vastly improved the performance of the "scan and scale" photo screensaver and significantly speeds up iTunes visualizations. But on more prosaic things like launching applications having it has positively *zero* effect.
(As for increased program compatibility, well, there's not much that runs on Tiger that *needs* a G4. Having it lets my B&W fire up Garageband, I guess, but it's still too slow to really use it. I wouldn't consider it much of a benefit.)
Was the improvement of those scaling/GUI functions more than a 25% clock speed bump would provide? Probably. Almost certainly. Just beware that if you use non-Altivec-aware programs the machine *will* actually slow down, as for "normal" instructions the G4's of the ZIF vintage don't seem to be *at all* faster than the G3s per-clock. So you might be trading, say, faster word processor page rendering for a faster GUI and iTunes.
Since you've already bought the G4 you might as well experiment and try it. Removing the firmware block won't prevent you from going back to the G3 if it better serves your needs.