Do you honestly think Blizzard will tune Cataclysm instances the same way they have in WotLK, thus making normal modes comparatively harder for the 10man raid groups because they will no longer have the benefit of gear from a higher level? No, they won't. Blizzard will not make it harder for those groups and by extension to achieve some semblance of parity between the difficulty of 10man and 25man they will have to tune down 25man quite a bit making 25man even easier than it is now.
Yes I do, but anyway it's not really a matter of beliefs. Wrath raiding history so far demonstrates that they can tune 10m very tightly with respective gear. The fact that most get a boost by raiding 10m content with 25m gear causes dissatisfaction among 10m guilds. In fact, if anything I hope 10m difficulty will get more respect when it suddendly can't be beaten as easily by people in 25m gear, thus increasing their perceived challenge level and consequently the fun derived from doing it.