I've always wondered what people expect to get from a game like this. GTA-style game are popular and fun not because you can drive a bunch of vehicles there, but because it's an open world filled with things for you to do. Stripping away most of the mundanities of every day life is what makes these games so accessible and fun, that includes the simple driving model.
Imagine that you're in the middle of a difficult mission with the cops all over your tail and another gang shooting at you from a truck. Driving, shooting, avoiding traffic, accomplishing the mission goals and forming an escape plan is difficult enough already, now imagine that you'd have to take care about changing gears, overheated brakes, tyre hotspots, running out of fuel etc. on top of the raw unforgiving LFS tyre model. I'm of the opinion that such a game would quickly turn from fun to extremely frustrating even for people who have thousands of miles in LFS under their belt.
Hardcore racing sims do not make up a large market, narrowing it down even further by crossing it with a GTA/Mafia shooter would probably make the target audience so small that it wouldn't justify the production costs. Following this logic I might ask for a Cessna 172SP simulator with advanced flight model, ATC and stuff just so I could mess above the Liberty City.
Don't take me wrong, I'd like to try such a game once but I'm down to the Earth enough to see why it's not going to happen anytime soon...