I run 2.5" all the way from the compressor to the IC (which has a 2.5" inlet/2.5" outlet) and have to lag problems at all. The outlet on my t4 compressor is 2". I have a 2-2.5" reducer on there attached to the IC pipe going to the intercooler. The way I see it is that the less resistance getting the air to the IC is the less the IC has to work to cool it down. I.e., compressed air raises inlet temps. Now I am not talking about massive piping because you want to keep the velocity up as well. 2.5" piping is a good size for our cars making the power that they do. IMO I would just leave the piping the way you have it.