What engine is currently in your girlfriend's car? Is it a B16? D16Z6? You said "Si motor", so I'm figuring it's one of those two. If it is a B16 and you've got it hooked up to an automatic LS transmission, that's part of the problem. That transmission is meant for the LS engine (i.e. torque down low, no VTEC, so no power boost up top, therefore the gears are long). Hooking it up to a GS-R engine might help a little, but it still isn't going to make it much faster.
If it was me, I'd sell the engine and start looking for an automatic GS-R transmission (they made them in Japan). They're a little rare, but can be found if you search. A car's ability to accelerate isn't all dependent on horsepower. A transmission with shorter gears can make a big difference...