The RS5 is gonna rock- no doubt about it. 450hp is not a ton but it is ample for the car. Remember the rs5 will be placed between the rs4 and the rs6 in price and performance. Audi can't have a cheaper rs5 walloping it's more expensive rs6.
And it is true that the 155mph limit is for all german cars except porsches. It is possible to get them de-tuned and this car would probably make 175 mph is my best estimate.
The interiors of audi are very very nice, I would say the best on the market. I've been in many A4's, two S4's, an RS4 and an R8 (yea that's right). I've also been in many mercedez and many bmw's --- my dad get a new lease from his office about every two years. The audi interiors have always been beautiful.
If I could get the money I would definitely buy an RS5-- youtube s5 exhaust and it sound amazing. I can't wait to hear what the rs5 will sound like.