I'm running a full 3.5 inch exhaust system (DP, Hi-Flow Cat, Cat Back) and I decided to slap on the silencer on the exhaust the other night and for some reason the car felt alot smoother to drive in terms of gear change and when decelerating etc.

Does this have something to do with backpressure or am I totally imagining things?

I also took a drive in an 2000 RZ which had the stock exhaust system and again the drive was very very smooth similar to the feeling I had with the silencer on.

 

Yes, backpressure can have that affect.