Best practice: Homeplug AV Adapter Kit vs Adapter+Switch

Hi,

I have question for general discussion and best practice topic. I am using HomePlug for almost a year now. Generally I am very happy with it. I am getting about 200Mbps max while copying big files.

I tried 3 D-Link configurations:
1. Dlink NAS - cable 1m - Dlink router - cable 15m - Dlink Homeplug 500 adapter - powerline - Dlink Homeplug 500 adapter - cable 1m - computer
2. Dlink NAS - cable 1m - Dlink router - cable 1m - Dlink Homeplug 500 adapter - powerline - Dlink Homeplug 500 adapter - cable 1m - computer
3. Dlink NAS - cable 1m - Dlink router - cable 1m - Dlink Homeplug 500 switch - powerline - Dlink Homeplug 500 adapter - cable 1m - computer

I can see differences with blind eye:
1. 150 Mbps max
2. 120 Mbps max
3. 200 Mbps max

It seems like Powerline switch in the middle of my flat is most effective configuration.

Questions:
1. Why?
2. Why adapter in the same location as switch is performing worse. Is that a problem of adapter? Could it be QoS in switch?
3. Should we wait for AV2 switches?
Products used: DHP-540, DHP-501AV, DIR-857.

Thanks.