Well, if you sideshift, hit a turbo, and use a speed pad, I think that will allow you to hit even higher top speeds than just turboing and using a speed pad because it will add the sideshift speed (I think about 250 km/h) to your speed when you perform this (This was also a counter to the sticky stuff at Anulpha) technique. However if there is a speed cap, we would need to launch 2 different crafts somewhere, spamming turbos, sideshifting, and hitting speed pads. If both crafts hit an identical top speed, then we'll have our answer. As I have said in my previous reply, we could also glitch the game and send a craft flying off the track and continue falling endlessly until it hits a top speed. If it rapidly accelerates and then suddenly stops accelerating, then there is a speed cap. However, if it starts gradually accelerating slower until it stops accelerating, it would most likely air resistance has stopped it from reaching any higher speeds.
Hope this helped,
~Mike