Wow this was a lot of effort somebody went through. My (utterly useless) thoughts:
General: FH1 has held up really well*. The sounds in that game feel a little more honest than its successors and the engines have more audio differentiation/character throughout the rev range, the succeeding games sound a little more homogeneous to my ears, like PG switched to an audio model that used fewer samples or something. FH4 was the worst in this regard, almost all of the cars sound "blurry" and indistinct, like they just pitchshifted a single sample to cover most of the rev range (listen to the Stratos in the video from FH4, the upper rev range just all sounds the same, just pitch shifted) - this can make cars sound super phasery at high RPMs, especially when highly tuned.. I can only fault FH1 for some sometimes-noticeable looping, noticeable sample transition, and lower bitrate samples. I feel like they kept adding effects and it gradually ended up in the audio mess of FH4.
Supra - I think FH1 actually sounds the most correct and there is a lot of that characteristic 2JZ burble in the exhaust. The FH5 example seems to be highly tuned...I'm curious to know what the stock supra sounds like. I don't think the FH5 Supra shown actually sounds particularly good...
Stratos - FH1 has more subtle detail than FH2-FH4 which sound too "synthy" to me. FH5 is good here.
BRZ - FH2 and FH3 get it right for accuracy, the rest of them sound incorrect. (equal length vs unequal length exhaust manifolds)
I still think the F355 sounds bad in FH5. I think FH2 or FH3 sounds more accurate.
*Anyone ever remember the Skyline GT-R 2000 in that game? It sounded absolutely sensational:
All in all, I'm very glad that the cars for the most part have audio character back - they have righted the ship. FH4 sounded so completely flat and lifeless which really took the joy out of the game for me.