I believe i have a 40 tooth rear sprocket, if i got a 38 tooth would i notice a difference in acceleration.

 

38 is stock, someone changed the rear sprocket out before you bought it from them. Since these bikes are alreay geared low from the factory, that has been worsened by the 40t you say you have. Stick with the stock gearing unless you intend to do some more mods to the engine.