OutRun isn't even a great example, it was manufacturered with a defective sample rom so will sound a muddy on the percussion regardless. (This is confirmed by many sources, including Sega directly)
Holly cow I never even knew this, so outrun should sound even better than it does??. Haze is there anyway to 'fix' this in mame and possibly have it so the sample rom sounds like it was supposed to.....would be really interesting to hear what it was supposed to sound like?
There's an unofficial fixed rom about, I believe HBMame supports it (I forwarded it to the CannonBall author who included it in the enhanced edition for original hardware.
http://reassembler.blogspot.co.uk/2011/08/outrun-enhanced-edition.htmlbut yeah, the MASK ROM was manufactured with 1/8 of the data lines stuck high (typically a dumping error but in this case a manufacturing error) if it was a graphic rom instead there would have been lines all over the screen, but because it was an sample rom it went unnoticed for years, even the OSTs and soundtracks in later re-releases were recorded with the bad data, it wasn't until much later they noticed, there's an interview that mentions it somewhere.
actually if you read everything that's on the CannonBall page you'll see just how many little bugs, flaws, and 'works by chance, so ship it' bits there were in OutRun!
Of course MAME uses the real (defective) ROM because that's how the game was, but yes, you can get better sounding precussion etc. with the fixed rom.
more info here (including visual comparisons and sample recordings)
http://reassembler.blogspot.co.uk/2013/01/outrun-original-game-shipped-with.htmlre: Double Dragon, it did actually slow down a bit too much in older versions, and is improved now, although it does still slowdown quite considerably, especially with 2 players.