it adjust more than just the boost. it chages all the air parameters as well.
a high quality kit such as carbonio will give you around 7-8 bhp if your lucky. and set you back around $2-300. to be honest its proberly better fitting a high flow panel filter and fit a large cold air feed to it.
a full turbo back systems quites good on itas well.
jus noticed the age of the car,. make sure its been well serviced, take the oil sump off and check the pick up pipe for any clogs