On my bike (a cheap hybrid) I get about 1-2 mph faster on flats after loosing 21 kg(that's about 44 pounds) from 120kg to 99kg. On the other hand I am a lot faster on climbs. I will post again in 3-4 months when I (hopefully) get at my normal weight(81 kg).
I do think that since you are already carrying 5-10kg in excess weight you really should not pay 2000$ more for a lighter bike. On the contrary heavier is better (already wrote this on the MTB vs. road thread) if you want to loose weight.
If you are into competitions of course you need a light bike but I think that most guys on this forum need to loose weight first and then get into racing(I know there are exceptions).