I have struggled for months to get through this mission (which involves fighting your way out of a SWAT bust). While getting in and fighting on the way out was relatively easy losing your wanted status seemed impossible.
In the end I used the following tactics: travelling to the mission I had the chance to get a Range Rover style chunky SUV, this was handy when escaping because they can take an extreme amount of abuse.
I also tried to avoid killing the gangsters, instead sneaking around the back of the building and climbing up the ladder at the back before sneaking into the room with the drugs. This means the gangsters and the SWAT will be shooting one another and will avoid pushing up your wanted level.
When escaping I started off with a shotgun and tried to leave the building very rapidly, switching to the SMG when entering the entrance building. This was to avoid more beat cops arriving to join the raid.
Jumping into the SUV and weaving out of the car park I turned left back onto the overpass to the city. This seems to be the key to losing the police as the SUV is slower over the straight than the police cars so you need to be using the grid system to make lots of turns and twists to avoid pursuit. Once the helicopters have lost you (zipping around tall buildings helps) this becomes the same job as losing any pursuit.