The fan and burner of the WH are designed to provide a certain airflow. Too much restriction, and it can't move as much air which could affect the heat transfer and safety of the exhaust. PVC can only get so hot before it has structural problems...slow the airflow, and things get hotter. It may also affect the fuel/air mixture at the WH. Exhausting into a corner would also prevent the diffusion of the heat, moisture, and CO that will be in the exhaust. The manufacturer knows its product. I'm sure there's some margin for error, but you've exceeded it, maybe more than a little. I doubt anyone here in good conscience will suggest making it worse by extending your installation. If the install was inspected, one of the criteria is that it be installed per the manufacturer's instructions, plus any local and national codes. It should never have passed inspection.