I happen to know a car that went BACKWARDS when going from a 3" to a 3.5 inch.
anyway, I repeat, the smallest part of your exhaust is the most restrictive. Be that your dump, a flange, bad muffler etc.
My Fujitsubo Power Getter was 3" from the cat to the second section where it flared up to 3.5 inch. My reasoning is that the most restrictive part of the exhaust is the first section where it is 3", and where the gasses are "biggest and hottest". I changed the rear section to a 3" muffler and I dropped about 8dB, made my car legal and it still made the same power as before.
Doesn't mean I don't put on my big pipe for when I want it loud though