O.k., you purchasing new wheels, know the specs but not sure exactly how much it will stick out or not.
offset - good explanation found here http://www.tirerack.com/wheels/tech/offset.htm
"The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel"

so.. lets use 2 wheels

1) stock wheels: 15x6 inch + 45 offset | 6inch = 152.4mm

152.4mm/2 - 45mm = 31.2mm = distance from hub mounting surface to outer end of the wheel
152.4mm - 31.2mm = 121.2mm = distance from hub mounting surface to inner end of the wheel


2) rota slipstreams: 15x6.5 inch + 40 offset | 6.5inch = 165.1mm

165.1mm/2 - 40mm = 42.55mm = distance from hub mounting surface to outer end of the wheel
165.1mm - 42.55mm = 122.55mm = distance from hub mounting surface to inner end of the wheel

next,

122.55mm - 121.2mm = 1.35mm = this is by how much rotas will extend in
42.55mm - 31.2mm = 11.35mm = this is by how much rotas will extend out