The STI was modified with a few things, and is probably a foregleam of the next limited edition S20x STI. The stock car in the game won't quite match it.
We already know what it was modified with
Subaru EJ207 engine from a 2010 JDM Subaru Impreza WRX STi R205 with larger turbo producing 320 bhp (239 kW; 324 PS)
The car starts with the suspension from the improved-for-2011 WRX STI—it’s 5 mm lower than the 2010 model, with 1-mm-thicker front and rear anti-roll bars, higher-rate springs all around, stiffer rear-subframe bushings, and new front-suspension pillow-ball bushings.
Weight is saved through the use of an aluminum hood from the spec.c, unique aluminum front fenders, the spec.c’s smaller battery, and the deletion of the radio as in the R205 and spec.c.
Extra aero parts specific to this car include a full undertray, a front-lip extension tacked onto the R205’s lower spoiler, and a Gurney flap added—taped, really—to the rear at Mäkinen’s request.