Hi Matt - welcome to the forum!
The ARC Air upgrade kit should work absoutely fine with your GT40 Sport set. The track layouts in the app don't add much to the functionality, certainly not to the pace car function - you set the pace of the car manually to suit the car and your track, however long it is. The only thing I can see is that the track length will determine the speed data - which is not something I ever look at anyway... The simple fix is to pick a track about the same length as yours.
I really like ARC Air. The wireless throttles are fab and using the app pushes 'standard Scalextric' analogue running about as far as is possible. The only thing to say is to be aware of the Android device compatibility issues - the list of compatible Android devices is small. However, most post-2011 Apple devices work well and the 2016 and 2017 Amazon Fire tablets work well with their own Fire OS version of the app. There is also the unofficial Magic ARC App available, which will be worth looking at for a slightly different pace car feature.