Got to play around with the modified APKs and web tool today.
Big thanks to those who made it possible.
Basically created Route X on drugs - its 18.9 miles long, 80 meters wide and has 9968ft of elevation with a handful of jumps on the way down. Spent a couple of hours messing around with a bunch of road cars, where 650-750mph seems to be the norm for most of the powerful cars. Got this in the Supra.
I'm sure I'll tweak the track to try and hit 999mph with a road car, but until then I will be tailoring proper tracks without the limits of the unmodded app. After some screwing around, I see little reason for some of the restrictions [90 degree corners etc]. Haven't had issues with scenery or anything like that when I was exploring my curiosities.