Discuss and learn French: French vocabulary, French grammar, French culture etc.
Hi Tony -- as far as I'm aware, you'd only get a difference in emphatic speech or where you're deliberately altering your pronunciation (e.g. in dictation) to emphasise the "l'". (It would be interesting if some data did show that speakers made a subtle difference, but I'm unaware of any.)
I would say there may be a little more stress or "emphasis" on the "l" in l'avez than lavez. You have to think of it like this. L'avez is technically 2 words. So, as a way native speakers may distinguish the l'avez and lavez is buy adding a little more "emphasis" to the "l" in l'avez.
However as a non-native speaker when talking to a French person, don't sweat the difference. They know what you're trying to say.
Actually, some models would say that l'avez it's "technically" one word -- that's really just a question of your model and how you define "word".
But either way, there's no a priori reason to expect more "emphasis" (whatever that really means phonetically in this case). There might be, and it would be interesting to see data that showed this. But in the absence of actual data, I see little reason to assume that there is.