Exhaust help por favor.

ok my situation is i have hooker lt's under my bed. My plan was to sell my LM and custom fab my own exhaust up. My dad told me i had to cat it and it be legal os thats out. Now i havent told them that my lt's arent legal yet and im sure he will be hell a pissed when i do. So i dunno what to do. When i got my car inspected last time they did not look under neath not from what i saw anyway so with lt's and if i catted my y can i pass a sniff? Thats another reason i wont be able to make my custom cuz it was my own form of true duals so thats out. So i guess im keeping my LM. The main problem i have is with a dual catted y will it deminish my gains of power? More importantly to me at this point will i lose my sound?