The manual states 93 octane because not all blends of gasoline are the same, ie cheap crap and a premium grade. This way you are covered if you buy el cheapo gasoline that is not blended good or has contaminents that cause a lower octane number. My car has a better exhaust and intake, so my engine runs a little more efficient than when it was blowing with stock components.
HP is a relative measurement for work done by something. Engines are tested 3 ways and are as follows:
1. BHP (Brake Horse Power) - This is also known as HP at the flywheel. It is strictly the ouput of the engine itself minus any accessories, drivetrain, and exhaust system and is measured on an engine dyno.
2. WHP (Wheel Horse Power) - This is a measurement of the total output of the engine thru the drivetrain with all of the accessories on the engine to the wheels. This is a more accurate degree of the output power of your motor/tranny combination as a whole. It is measured on a 2-wheel or 4-wheel dyno.
3.BDHP (Butt Determined Horse Power)
- This is a factual measurement take by the seat of the pants. Although, a marked increase in torque and HP figures will translate in an exponential increase in the BDHP, most are rather inaccurate readings. Numerous BDHP readings are skewed towards the tester's eagerness for his/her ride gaining performance after the install of speculative components. A common example would be the jubilant glee that one has after installing their Turbonator!
Note: To measure the total overall performance of the entire vehicle would require a 4-wheel dyno in a wind tunnel.