De acordo com o site do Pathfinder SRD,
These bonuses can be added to the weapon, stacking with existing weapon bonuses to a maximum of +5
Então você pode pegar a arma +1 depois do feitiço Arma Mágica e adicionar +4 de bônus a ela do Bond. Então você não vai superar o bônus até o nível 17, onde eu imagino que você terá muito melhor do que apenas um nível 1 Magic Weapon Spell.
EDIT: eu esqueci de abordar a questão do DR. Nesse caso eu faço referência:
Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters.
Também só para ter a capacidade completa escrita:
At 5th level, this spirit grants the weapon a +1 enhancement bonus. For every three levels beyond 5th, the weapon gains another +1 enhancement bonus, to a maximum of +6 at 20th level. These bonuses can be added to the weapon, stacking with existing weapon bonuses to a maximum of +5, or they can be used to add any of the following weapon properties: axiomatic, brilliant energy, defending, disruption, flaming, flaming burst, holy, keen, merciful, and speed. Adding these properties consumes an amount of bonus equal to the property's cost (see Table: Melee Weapon Special Abilities). These bonuses are added to any properties the weapon already has, but duplicate abilities do not stack. If the weapon is not magical, at least a +1 enhancement bonus must be added before any other properties can be added.
Portanto, seja qual for o bônus final + X, o desvio de DR é necessário.