Well, if Viewtiful Joe is any indication, it'll have framerate drops, lower-rez textures and slowdown. Coverting a game between two different platforms isn't magic. There are things PS2 does Gamcube doesn't and vice versa. Keep in mind Capcom isn't EA - they don't alway use a base platform and build it to the lowest specifications possible so it can run on all playforms.
RE4 was built for GameCube, it will look and play better on GameCube. Metal Gear is a series built for Playstation 2, even if the Xbox version of MGS2 Substance can give me higher-polycounts, PS2 still does various filtering techniques better and the PS2 will perform better on PS2 becasue it was made for PS2. Full Spectrum Warrior is best on Xbox because it was made for Xbox, the PS2 version just won't be as good-looking.
What ever content/levels they throw at RE4 for PS2, the GameCube version will still be better, not to mention the impact with gamers has already been made and the buzz will be near-dead once the PS2 version hits. Lots of people said it was smart to move VJ on PS2 to get more sales, but the increased sales never happened.
Why? Because everyone who wanted to play VJ (which, unfortunately, isn't many) had already done so on GameCube. Adding Dante didn't do shit.