I run a dropin K&N in the stock airbox with the top of the air box off, swiss cheese mod on the bottom/sides of the air box, silencer box delete, and a 75mm ported TB.
I've read on FSC that the GM tophat reduces the airflow by 4% as compared to the K&N FAIK. So I'm sucking in 4% less air than the Volant/K&N/Spectre kits, etc. I've spent some time gathering parts to replace the stock GM tophat with better flowing ABS piping, but the snow is here, and I'll wait until spring now.
As far as heat soak goes, I'm ok with that too. I monitor my air intake temps, and after 30kph(18mph), the air temp drops to ambient air temps. The heat soak only hurts when taking off from idling and even than it's only 20degrees more.
So far, I'm happy, and I can live with the limits of my system and I'm happy I saved $$$.