if the exhaust piping sizing is too large you will def loose torque however you will gain abit of topend speed. is really depends on what you like. but getting a reasonable size exhaust is recommended, not too large.
hey u said u have no cat? isnt it illegal to not have a cat installed?
u can get better front brakes, for the conversion rear disc brakes its not goner be so easy.
good luck on ur car though, i love a modded civic coupe.