Not having a go but that isnt really possible i feel that the improvement you were thinking you felt was in your head. Unless ofcourse the intercooler was a retriction to the power you were making.
Look at it like this, with the sidemount the volume that has to be filled before making boost is very small. The standard return pipes and the intercooler itself. Then compare that to all the piping and the 600x300 intercooler that needs to be filled before you will achieve boost after installing a front mount intercooler. Low down response will deffinately suffer as a result. It may not be a huge decrease in response but there will deffinately be a decrease.