I hate to be the bearer of bad news, but whatever salesperson told you you were making 18 more horsepower was likely pulling your leg. Typically, a full exhaust system is only 5-10% power gain, max. 25+% is unheard of. It just doesn't work that way. You're not generating more engine power, just freeing up air flow and no stock exhaust is that restrictive. Until there's a dyno sheet to back it up, cited hp gains are largely useless, but hey, you didn't buy your bike to go fast, right?