Presumably (and by that I mean what the others have said, that we can't be certain), the +5 is meant to be a hard cap, like the +10 modified bonus for enhancements and equivalent enhancement from abilities on weapons and armor. So the item could grant a +2 enhancement bonus to attack and damage and have keen (+1) and flaming burst (+2) for a total modified bonus of +5.
As for whether temporary effects can raise that above +5 modified, it would seem to be No.
Magical Weapons wrote:
Some magic weapons have special abilities. Special abilities count as additional bonuses for determining the market value of the item, but do not modify attack or damage bonuses (except where specifically noted). A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents, including those from character abilities and spells) higher than +10.
The wording of the item you quoted seems to be pretty boilerplate and identical (other than as noted). From the reading, it would appear that temporary enhancement increases won't work if the item is already +5 (or would go over it). Such instances might include a magus increasing their weapon's enhancement bonus with their arcane pool, or a high-level caster trying to use greater magic weapon on a low enhancement but ability-heavy magic sword.
From the reading, this may or may not also prevent a bane weapon from getting it's +2 higher enhancement to attack and damage against its specified foe if it would take the bonus higher than +10 (or +5 in the case of your item), but that's just from a quick reading. The provided quote doesn't mention weapon abilities, only character abilities and spells, so your GM may have to make their own call on such situations. This may or may not have been the intention (or even considered) by your item's author.
