Yea, the problem with such a fast motor is that you're going to get a TON of wheelspin when you accelerate. The tires will spin so fast that they won't be able to grip on a loose surface...
As for the straight line, Speedo is correct, check the radio and make sure that your left/right steering trim is adjusted correctly. If a manual came with the radio, the instructions for it should be there. Keep adjusting the trim until the car goes in a straight line when driven slowly. At high speeds, the car may drift if there is uneven weight distribution, so if that happens you may have to add weights somewhere, unless it's a more serious problem.