Concerning IV DS: The difficulty is indeed increased, at least to the level of the original Japanese, if not more. The bosses are also stronger and use different strategies/have different abilities (for example, Octomammoth's attack increases with each arm that disappears, and Scarmiglione will counter attack with Gas, Poisoning, Silencing, Blinding, and putting to Sleep everyone).

I would definitely say that an increase in difficulty is in order for a VII remake, no question.