The engine in the AE86 in question is a Toyota Formula Atlantic engine out of a formula atlantic car. In real life, it is a highly modified 4A-GE with 13.75:1 compression, 304 degree cams, individual throttle bodies, the works. The car is mad tuned. If you could reproduce the work, and thats a big if, it would be very un-streetable. So therefore, it looks good as a work of fiction. Now, if in fact you could get one of these engines(fat chance, unless you build it from either the closest HKS parts you can find or you happen upon the real TRD parts...) and shoe-horn it into an AE86(its hard enough getting a 20 valve 4A-GE into one, but it can be done) and you had anywhere near the suspension tuning needed, it would be hard for a lot of cars to beat you. 250hp, 11k rpm redline, who could beat you? That is if you didnt get pulled over first from all the noise. Deafeningly loud. So loud that they are about one of the only formula cars that have to run a muffler of some sort.
So, do I find it hard to believe that it beat the Miata in the downhill? No, because the Miata, while a great car, did not have HALF of the engine that Takumi had under the hood. And neither did the EK9 CTR. Though, in terms, the EK9's engine is about the closest to Takumis tune wise out of all of them so far.