GT30 with 0.63 a/r exhaust housing come on like a light switch??? YES very much so. You will find it very hard to get off the line in the first few gears with that small housing and you will be in wheel spin city, and then have no power high up in the rev range. And if it raining forget about driving your car.
My GT30 with 0.86 rear and 0.70 front housing (RB25 internally stock) comes on around 1 bar at 4000rpm and makes power all the way to 7500rpm, and bounces off the rev limiter in every gear quite easily, and is easy to drive around normally and come on power nicely without to much wheel spin.