i drive an automatic 91 honda accord and i wanna get a performance exaust for it
will it make a difference that im using an automatic?
or will it be the same as a stick shift?

A stick shift will walk away from it regardless but a performance exhaust will help most cars auto or not.

I have a 90 accord auto did a full cat backexhaust system on it i notice now that i have a quicker picklup off the line also it seems my 1st and 2nd gears i go thru them pretty quick i would recommend you do the exhaust seeing how it did help me