The 8% is a number IRS came up with to take into account several factors.

1) Not everyone tips
2) Waitstaff often have to tip out from the money they get in tips (they don't get the full 15% you leave them)
3) They are getting paid 2 to 3 dollars an hour some of their tip money goes into making sure that they make at least minimum wage as required by fed law
4) In fancy restaurants it isn't uncommon for people to subtract the alcohol from the total bill before they tip. Which doesn't bother me. As a former waiter I never felt like I deserved an extra $7-8 because someone ordered a $50 bottle of wine. However, most restaurants base their tip outs on the total sales as does the IRS.

Obviously you are free to leave whatever you feel like, and I wouldn't feel obligated to leave 15% on sub-par service. Just remember that most full time waiters make around 20-30k in my area. So it isn't like we are living in mansions rolling around in the money that you leave behind for us.