It is a special modification that can be carried out. It converts your car into a racing car, with a racing bodykit, a stripped out interior, and a slight power boost. The cars that can be converted are:
'91 NSX
'69 Camaro Z-28
'10 Camaro SS
'06 Corvette Z06
'09 Corvette ZR1
'70 Challenger R/T
'97 Civic Type R
'04 Integra Type R
'07 IS F
'96 Elise
'04 Elise 111R
'05 Lancer Evolution IX
'02 Silvia Spec-R Aero
'05 Impreza Sedan WRX STI
'95 Cappuccino (NOT the '92 as I once thought...)
'00 Tuscan Speed 6
'01 Golf IV GTI
I just know someone will have posted the answer while I was writing this...