I think this depends on how you're answering the question. It can be interpreted in two ways 1) Could Alexis Denisof have originated the role in WTHM and how would things have been different? and 2) Could Wesley, as we meet him in BGs, ever establish a working relationship with WTHM era Buffy that would have resulted in her working as and developing as the slayer she becomes?Look back at Giles in the first couple of episodes of Season 1, and he wasn't a whole of a lot more competent than Wesley was in Season 3. Okay, that's somewhat an exaggeration, but not too much of one. My point is, if Wesley had been Buffy's original Watcher in Season 1, he would have developed and matured just as readily as Giles did.
I like in Orpheus how Wesley calmly and non-judgmentally asks Willow how she plans to get Angel's soul to cure him. It's more collaboration and "checking her work" than Willow got from S2 through S6 with Giles. Giles's MO wasn't to ever ask Willow about the mechanics of her spells but instead, to just expect magic and scold her/demand car-detailings if it wasn't the spell or success level he wanted.Just imagine, Season 4 "Angel" Wes as Buffy's Watcher in Season 3. Faith accidentally kills the Assistant Mayor, and what happens? Buffy and Faith call their Watcher, and he comes and helps them dispose of the body, then they all go to the Bronze for a cool (nonalcoholic) drink and a discussion on ways to better avoid collateral damage to bystanders in the future.