Actually. If you looked further into it, you CAN do a bit with that transmission. Take the car to a transmission shop and they can uprate everything in it. The 4T45E was also used in the following popular tuner cars:
1999-2005 Pontiac Grand Am
1999-2004 Oldsmobile Alero
1997-2003 Chevrolet Malibu
1997-1999 Oldsmobile Cutlass
2000-2005 Chevrolet Cavalier
2000-2005 Pontiac Sunfire
2005-2007 Saturn Ion
2005-2010 Chevrolet Cobalt
2006-2011 Chevrolet HHR
2007-2009 Pontiac G5
That's a short list, but that was GM's automatic transmission *&&^$ for the longest time. If you truly want to switch transmissions though, GM's 4T65E, and 4T65E-HD are great choices with plenty of aftermarket. However, you would have to worry about transmission bolt patterns at that point. Being an L300, it uses the 3.0L DOHC V6 Cadillac used in the Catera, and Opel/Vauxhall used for years in the Vectra and Omega. It's part of the 54* V6 engine family and I don't know if it shares the same bolt pattern as any of the cars which used the 4T65E or 4T65E-HD.
A better options would be switching to a manual transmission, but this is a big job, requiring pedals, ecm/pcm, axles, transmission, shifter, wiring harness, etc. GM's manual transmissions can take a beating. The Getrag F23 5 speed in my Saturn Ion is rated to 175 ft-lbs, but the transmission itself can handle up to 300 HP no problem (proven many times over in the Chevy Cobalt and Cavalier community). The torque rating on that manual is more for the clutch than the transmission. A big job for sure.