There's a massive gulf between 'World Class' and 'Giroud level'. All this 'world class' talk is largely nonsense anyway. Teams have won titles and champions leagues without world class players. Football is about systems of play, individual strengths and maximising efficiency of those systems and strengths. When it comes to Lacazette his strength is clearly his finishing and his shot alongside a decent all round game. His movement is disapointing and I'd like to see it improve. But if he has a chance more often than not he'll put it away.
Also with our current front 3 Lacazette can play behind Giroud or in front of Alexis/Ozil.