Installed new exhaust on my GS460 2010 and been told the power gonna increase but instead lost power and speed. Have no idea what the problem might be

 

Did you replace the headers/ catalytic converters also. Either way any aftermarket mods installed without the ecu being tuned will not increase power and like you mentioned, you could even lose power after the ecu see's the change it will change it's adaptation to protect the cats/ keep the vehicle running for best emission. Sorry you kinda of got lied to