Dual exhaust - surprise increase or imagination?

[Jack 34pu]

I finally put duals on my 221ci 34pu yesterday. I drove it around town a bit and really didn't notice anything other than a sweeter sound. Today, I took it out on the 4 lane and I was shocked at the difference starting arounf 40mph and up. Is a fairly noticable increase in power and smoothness to be expected or is my mind playing games with me? Of course my wallet is somewhat lighter!