That would be good if the RMU system is consistent, and I like that approach: Pures have the lowest open/closed but highest arcane, Hybrid are in the middle, and Arcane casters have the highest open/closed but lowest Arcane. That's also consistent with RM2.
I should note that although the idea in Companion I seems to be that the Arcane spells are more powerful, in practice that was not at all consistently applied. E.g. The Manafires list in Companion I had Fireball one level earlier than the base Magician list did, but it also had Woodfires and Wall of Fire at the same level. The main advantage Arcane lists had in the Companions was that they allowed characters to do unique things, like inscribing Bladerunes and making constructs/golems. So in practice, Arcane spells' main advantage was a wider range of spell lists to choose from, which did unique things, at the price of higher costs and casting restrictions.
Now that RMU has shifted constructs and bladerunes and such to specific (traditional) casters, namely the crafting professions in Treasure Law, that Arcane niche isn't quite so clear. So that's why I think there is still some room to make Arcane spells more distinctive in the way I suggest above (e.g. they come earlier on the list, have heightened effects, last longer, have unique effects, etc.).