I know there are a few retail games that use various non-standard opcodes. I'm guessing those are supported? Are all the opcodes supported? Any differences from a standard NES?

I'm clearly in no position of specific knowledge or authority to say so, but I would definitely think so. As the FPGA in the AVS is supposed to electrically duplicate all of the original components in the NES, it should handle anything that the original NES can. There have clearly been some oddball exceptions to this (Xevious, Pokemon Yellow & FF7 being particularly notorious from what I recall reading), but 99%+ of everything seems to run just fine. What games were released with non-standard opcodes? I don't mind at all to give them a shot after work if it will help answer this for you.

from the nesdev page: https://wiki.nesdev.com/w/index.p...

The use of unofficial opcodes is rare in NES games. It appears to occur mostly in late or unlicensed titles:
Puzznic (all regions) (US release November 1990) uses $89, which is a 2-byte NOP on 6502.
F-117A Stealth Fighter and Infiltrator also use $89.
Beauty and the Beast (E) (1994) uses a different 2-byte NOP ($80).[1]
Dynowarz uses 1-byte NOPs $DA and $FA on the first level when your dino throws his fist.
Gaau Hok Gwong Cheung (Ch) uses XAA ($8B), an immediate instruction. The game malfunctions after selecting Left from the main menu if that instruction is not emulated.
Super Cars (U) (February 1991) uses LAX ($B3)
Disney's Aladdin (E) (December 1994) uses SLO ($07). This is Virgin's port of the Game Boy game, itself a port of the Genesis game, not any of the pirate originals.
As hobbyists became aware of what these instructions could do, homebrew games started to use them:
The MUSE music engine, used in Driar and STREEMERZ: Super Strength Emergency Squad Zeta, uses the unofficial opcodes $8F (SAX), $B3 (LAX), and $CB (AXS) [2]
Attribute Zone uses $0B (ANC), $2F (RLA), $4B (ALR), $A7 (LAX), $B3 (LAX), $CB (AXS), $D3 (DCP), $DB (DCP).
The port of Zork to the Famicom uses a few unofficial opcodes.

Super Cars uses the instruction I was wondering about (but a different version). I suppose that would be a reasonably good test.

Is there any place to get this cart or a ROM that runs on flash carts / repro boards / common emulators?  Only found the unif version which has zero mapper info.

 

the first rom i found is the unif one with mapper UNL-43272. it runs ok in fceux 2.2.3 although i couldn't say how accurate it is. 

Turns out the game is only running that opcode because of a bad bankswitch. Doesn't matter if the emulator does the opcode wrong (like fceux) because the game doesn't use the result. Only emulators that quit/crash have problems. Quality chinese programming