I want to add Rear Speakers to my current 7.1 wired system.

For the simplicity of wiring, I'll looking at the M3 Wireless speakers for this additional pair,

My first question is: If I used the pre-amp outputs on the Yamaha receiver to connect to the M3 Transmitter, would I notice any sync/lag issues with these rears? The rest of the speakers are wired directly to the Transmitter.

Also, the descriptions states "The Transmitter-only option plugs into the Right and Left output of your existing receiver / pre-amp, and lets you broadcast wirelessly to your speakers. This option does not include the AxiomAir features." Is the "Transmitter-only" option something you can purchase, or do you just disable the AxionAir features?

My 7.1 System looks like this:
Mains = M80
Heights = M3
Surrounds = QS8
Center = VP150 V2
Sub = EP500 V2
Receiver = Yamaha RX-A3080

It's not clear if the "transmitter only option" is an option you specify at purchase or if it's included.

You cannot rely on wireless to be synchronized. I think this would be more audible with bass, say a sub, than surround speakers. You can reduce latency by locating the transmitter so it is within line of sight of the speakers.

Depends ...
But knowing a bit how these things work you will have some delay. How much depends on delay of the circuity/SW and what axiom does for buffering. If software and circuitry are designed well and there is little or no buffering for direct connections ... more than likely you'll be fine. If there is buffering it will depend on how much and the delay in SW.

I'd ask axiom ...

Axiom will tell you the same thing.

I'd go for it, and if it doesn't work, punt it back to Axiom.

Mojo, cost nothing to call and ask them ... they will know.
I'd be surprised if this was not one of the use cases for the air. That said ...

Yup...done that and Andrew confirmed what I said.

Why didn't you say that in the first place ... sounded like you didn't really know ... but you do ...
uhmmm, I think ...

I did tell you. I said Axiom will tell you the same thing. You just didn't believe me because my nickname is a penny chew.

Nonetheless, if I really couldn't wire, I'd try it and maintain line of sight.