hi! I have a 98 base. first off, these are two ton cars with a full tank of fuel. the engine is only 281 cubic inches. it has not got a bunch of low rpm torque. they tricked it with the IMRC in the intake to help it make more torque at less than 3200 rpm. that helped. I would caution you not to do something that would reduce low rpm torque output. that mustang cobra intake looks sweet on the mark 8 but destroys low rpm torque.
what I have done helped driveability in traffic a bit more. I put a detroit trutrack differential and 3.73 ford racing gearset in the rear end. it requires some special tools(case stretcher) and measuring devices to accomplish the job properly. and no, it is not inexpensive. also replaced the driven gear on the trans speed sensor.this has worked quite well. the torque multiplication of the increased ratio-up from 3.08 on my car-definitely helps the car accelerate quicker. fourth gear cruise rpm is now 2200rpm @65mph which is right at the beginning of a good chunk of the torque curve and quite livable.
these cars never came with a limited slip differential from the factory. they used an open diff and traction control. traction control. it is a good thing in these cars when it is slick. I leave mine on because when it is dry with good traction, the rear end acts like a open diff, when it is slick it locks up solid like a spool diff and makes life interesting with the traction control turned off. so I leave it on.
If your outside of an emission testing area, modding the intake, exhaust and chips may works nice for you. I don't have that option so I went a different route. I kept the cats, cat back 2.5" dual exhaust. it's not a "raceable car" but it does keep up with the flow of traffic in the rocky mountains where I am.