Did you change oil in all those cars before checking their maximum tuned up HP?
In the cars that I have tuned, I have these HP and they're a bit higher
- Alfa Romeo TZ3 Stradale '11 (1223 hp)
- Aston Martin One 77 '11 (1239 hp)
- Bugatti Veyron 16.4 '13 (1343 hp)
- Cadillac Cien '02 (1060 hp)
- Lamborghini Diablo GT '00 (1071 hp)
- Pagani Huayra '13 (1250 hp)
- Volkswagen W12 Nardo '01 (933 hp)
By the way, your tests were on SSRX at the downhill part? To me that shouldn't count and neither would nitro, but each on its own.
I have tested it on my Audi R8 4.2 (973 HP), results are:
-Flat terrain, no nitro: 453 km/h (281 mph)
-Downhill + nitro: 486 km/h (301 mph)