I'm wanting to get a treadstone TR1245 but I don't think I will get enough air across it. With the stock G35 bumper, I know the most logical answer is to just get a new bumper but if I don't have to I would like to avoid that.

 

You're gonna need a new bumper to maximize the performance of the intercooler. I had a Nismo front (similar to the OEM front) when I first boosted my car. My coolant, intake, and oil temps were dramatically lower (when moving) after I swapped to a Kuruma front bumper.

 

Thanks!

 

an intercooler is only 80% efficient if facing forward, going at least 60 mph and is shrouded correctly.
just hanging one in front of the rad without shrouding it correctly will only be 30 to 40% efficient.
rather then going bigger just use the rule of thumb size (1 sqft / 100 hp for A/A) and shroud it correctly