Inter-coolers DO make the intake air more dense, as you specumolated, 
and thus more O2 for the combustion process. 
They are available for install, with some backyard engineering. The new TDi's
and the G60 have intercoolers as do many Saabs and Volvos. I have no
experience with aftermarket installs, or the quatitative metrics of
improvement in performace.

gmbulley
fairfax,va