Your post made me do a comparison between 576p and 480p. No pics, because I do not have any capture stuff, however, here is my research:
Testing setup:
-Gamecube with component cables.
-Tested 480p NTSC version as well as 576 PAL version through swiss iso loading with wasp fusion (I own both versions; Pal version was a leftover from my old collection before I decided to collect NTSC games exclusively)
-No added settings (such as widescreen mod etc.).
-Tested game is Mario Kart Double Dash.
To me it seems like there is an increase in vertical resolution only for the 576p version as opposed to the 480p version. I may post phone pics if anyone is interested, however, this article may prove more useful:
http://www.gc-forever.com/wiki/index.php?title=Swiss/Forced_Progressive_Compatibility_List
Now, this is were it gets interesting (I'm not 100% sure on this). I did a time trial test lap on Luigi Circuit with a stopwatch next to me. No AI present to knock me over, no mushrooms used which you get at time trial and, both test-runs involved me using the exact same cart/characters. Additionally no use of boost pads, not drifting and following the same driving line.
The NTSC 480p 60hz lap took me 33 seconds, while the forced 576p 50hz one took me about 39 seconds, a difference of 6 seconds. The PAL game at 50hz also felt a lot slower than it's NTSC 480p 60hz counterpart. Now, I hear you saying: ''well that is because you forced to display into behaviour it's not programmed to''. Well, I actually ran the (PAL) test lap again, this time not forcing 576p, just normal 576i and 50hz like Nintendo intended and guess what: it took me, again, 39 seconds. Also, 33 to 39 is the same ratio as 60Hz is to 50Hz respectively (when taking into account my slight variations in lap time).
So PAL 50hz mode, even during the gamecube era, is essentailly slower than NTSC 60: as in the gameplay is slower, just like in the old SNES days. Of course, the difference is that they did (correctly) adjust the speed of the music to sound identical in both 60hz and 50 hz, something we mostly missed out on back in the old days.
PAL 60hz plays fine (speed-wise), but trades the increased resolution for black bars on top and bottom. I always thought that 50hz
during the gamecube era was better optomized than on earlier consoles; it would drop 10 frames, but a second in the game would still be a second irl, regardless of hz and thus gameplay would not be slowed down.
Now, I may very well be wrong, or it may just affect one game (Double Dash), but I wouldn't be surprised if PAL 50Hz affects other games similarly. Would be interested in other peoples findings.
EDIT: I think I used a ntsc cube, alltough that shouldn't make a difference now would it?