Weight has very marginal effect on top speed. See for example this German graph for 109G speed decrease at various weights:
http://www.kurfurst.org/Performance_...fect_speed.jpg
The story short: the graph says that if you increase the 109G weight from 3000 kg massively, to 4000 kg, speed will decrease by about 7 km/h at SL and by about 18 km/h at rated altitude.
As more practical or realistic weight changes, ie. 1-200 kg (plane carried 300 kg fuel), the speed difference is only 1-2 km/h...