-5s & GT35 = basically the same thing.
That said Chris from Racepace did a 1:17 round Sandown in his car with -7s (330rwkw E85, full weight, no cage), which previously only cars with ~380rwkw could achieve. Goes to show that driver experience and car setup play a more important factor than outright power depending on the situation.
The only track huge power will help at down here (in a street rego'd type car) is Philip Island - stick with the -5s as is and learn to drive the car - and drop another 8k @ suspension/brakes/transfer case