Thanks @Eyedunno.
I get exactly the same timing for SF1 and SF1.2, the explosion of the ship when it is on the ground is spot on.
Yes, the credits are off, but the rest of the game is more accurate than any emulator or SNES-Classic, try snes9x credits. 
Funny. I tested it with 1.2 three times at lunch. Two out of three times, I hit the top of the arch thingy where you get the Twin Blasters. The other time, I died a little later than on the cart. I tested it just now on my S-video modded mini though, and the timing was indeed fine. I'll check on the Super Nt again in a sec.
Edit: Just checked 1.2 on the Super Nt again, and it too was fine. Must've been a fluke earlier, though the fact that it was three times in a row is sure weird. Oh well.
I will maybe check level 2 tonight.
I also have an early StoneAgeGamer SD2SNES from their first batch (#26) I can check later if I have similar issues, although I think mine is rev E2. I'll double check that when I get home later, too.
Thank you so much!
I opened up my SD2SNES, and it turns out I have a rev E1. Not sure why I thought it was an E2-- it even says "rev E1" on the sticker on the back of the shell...
I did a comparison of Super Road Blaster on stock 0.1.7e (top) and 0.1.7e-gsu-v08 (bottom):
https://i.imgur.com/z70jPoG.png
They seem pretty much identical to me.
Here's a quick video I put together comparing them along with Super Mario Odyssey MSU-1 (since it uses both MSU-1 audio and regular sound effects to show the difference between them):
https://youtu.be/HOK_s3atsFk
Thank you so much. That was a crazy amount of work, and you know what I hadn't bothered to check? The version on my regular SD card! It was a USB version of the firmware that I haven't used in almost a year and forgot I ever installed. Evidently the boost wasn't working right there, and I guess maybe my cart audio setting was too high on the Super Nt. The audio still clips at +12dB, even on an original SNES, but at least I know it wasn't the GSU-1 firmware after all.
Sorry to have wasted so much of your time on that.