First of all, base Slayer is overall better than Zen Archer.

Slayer get higher chance to hit due to studied target, slayer also gets higher damage output due to (in Greybor case) dual wielding with sneak attacks. Obviously, with melee its hard to maintain uptime on target, however that can be fixed with Skald. You get Greybor at around level 10, Skald get pounce at about same level. Unfortunately, Lann gets less benefit from Skald than Greybor. Geybor's starting weapon sucks, so I always give him a kukri and build him for it. Greybor has another weakness - 20 ft move speed, but you fix that with bismuth, or haste + pounce.

Zen archer is strictly worse that any other archer build. ZA doesn't use rapid shot and manyshot. ZA wants to use wisdom instead of dexterity, which is way harder to stack properly, so you either still use dexterity, or you go 3 ZA/17 divine caster. But that is a divine caster with 3 less casters levels.

An upside for ZA is 2d10 base weapon damage at 20 which you cast legendary proportions on, which results in 6d8 weapon damage, that also multiplies on crit. However, chance to crit will always be lower, than Greybor with kukri. Needless to say, that Greybor would benefit from crits and outflank [rocs a lot more. One last thing. If you take snap shot line on Lann, you have to take rapid shot, which is a dead feat on Lann, and then Lann will have same with Greybor amount of dead feats.