When you use a ki focus for a weapon attack, the ki focus' enchantment replaces the weapon's enchantment.
From PHB3, p203:
Being able to use a ki focus as an implement means you can also use it with your weapon attacks. You can add its enhancement bonus to the attack rolls
and the damage rolls of weapon attacks you make using a weapon with which you have proficiency. However, you must choose to use the enhancement bonus, properties, and powers of the ki focus, rather than the weapon you're wielding, or vice versa. You can't use both when you use a power.
When you use a power with the Weapon keyword, you have to choose whether to use your ki focus's enchantment or the weapon's enchantment for that power. If you choose the ki focus, then its enchantment effectively replaces the weapon's enchantment (not its other features) for the purposes of resolving that power.
For example, suppose you have a +5 Blazing Arc Ki Focus and a +2 Superior Crossbow of Speed. When you use a power that lets you attack with the superior crossbow, you can choose to treat it as a +5 Blazing Arc Superior Crossbow instead of a +2 Superior Crossbow of Speed for the purposes of that attack. It's still a superior crossbow, so it still has a 20/40 range, a +3 proficiency bonus, and the load minor property. It has a +5 enhancement bonus to attack & damage, does +5d6 fire damage on a crit, and can have its damage changed to fire as a free action.
On the other hand, if you use the power granted by the Weapon of Speed enchantment to make an attack, then you've already effectively made the choice: it's a +2 Superior Crossbow of Speed, because a +5 Blazing Arc Superior Crossbow doesn't have a power to let you make an attack.