Pikey arch mod - sealing from engine bay?

Has anyone messed around with the arch mod any more than just putting a vent in the arch? 

Before doing it, I saw a few threads saying the results where negligible, and that is about what I saw after doing it. Maybe 1-2°c less peak temps. (1.9 pd100) but what I did notice, that I'm pretty sure it now takes abit longer to get up to temperature. Not long, maybe half a mile/mile at 50mph. 

But that got me thinking that engine bay air, is now escaping through the wheel arch vent, which ties in with people saying they feel air coming out the vent when the Aircon is running. Leaving less 'space' for air passing through the intercooler to vent out. 

So has anyone sealed off the arch/smic from the engine bay? Looked at the engine bays in the vehicles that have vents as standard to see if the engine bay/arch area is different? 

I know I'm clutching at straws, but people always say if VW put the vent on X car, why didn't they put it on y car. Perhaps that is because there was abit more to it? 

Tear me apart if you want, but all just a thought in my head.