At 253mph the drag on a Bugatti Veyron is roughly (and I stress roughly) 1,352lb (613.5kg) - that's more than half a ton of air it's fighting through. To do that, it requires about 912hp.
(for those interested, the figures are):
Rolling resistance = 56lb = Weight (4,160lb) * 0.0135
Air resistance @ 253mph = 1296lb = Drag coefficient (0.355) * Frontal area (22.28 square feet) * Speed (253mph) * Speed (253mph) * 0.00256
Total resistance @ 253mph = 1,352lb = Rolling resistance + air resistance.
Power required @ 253mph = 912hp = Total resistance (1,352lb) * Speed (253mph)/375
Rather than working the numbers through again to calculate the new top speed, we can instead note that there's three multiplications of speed for every power - or that for every increase in speed, we need a CUBE increase in power... To increase speed 50% we'd need 3.4 times the power (1.05 * 1.05 * 1.05 = 3.375)...
So, if we shovelled 1,200hp at it (31.5% more power), we'd get 9.6% more speed - 277mph. An increase of only 24mph over the standard car for a near-300hp increase.