No issues here with the ground attack mission. Maybe check your controls config. The stall warning on the ground is likely due to tail wind. In real life, the aircraft is not supposed to be started in a tail wind but this becomes problematic in DCS. The developers have put in a fix which will be available in the next patch. As for the under-powered, the old flight model was probably quite unrealistic in how powerful the engines were so that's partly why the new flight model engines seem under-powered. That being said, apparently there is a Manual Power Reserve mode which is activated by the crank position on the starter mode selector switch. It's supposed to give an extra 400 lbs of thrust.