Its got the koni adjustable shocks from the factory. So both front and back stiffer then?
I'm pretty sure that the stock Boss 302 shocks/struts are actually made by Tokico. I'm actually reasonably sure that the stock GT shocks/struts are manufactured by them too.
If you don't know what dampers (shocks/struts) do then you really need to do some research as proper use of dampers will not make large changes in handling bias. It will change the bias at a particular portion of the corner (entry/apex/exit) but it will not make gross changes to overall handling bias. Dampers work by speeding up or slowing down weight transfer. If you stiffen up the dampers weight transfers quicker. If you stiffen just rebound it will quicken weight transfer AWAY from that end/side of the car. If you stiffen compression it will quicken the weight transfer TOWARDS that end of the car.
As to your question, I don't think we have enough information to really answer the question. When you say the rear is loose in tight turns, is it also loose in faster larger radius turns? Where is it loose at? Exit? Entry? Mid corner? What are you doing when it is loose?
Some people get this grand notion that they should be able to enter a corner and then plant their right foot and the car will just apply power. If that were the case the Mustang would be the greatest car ever invented. You are still subject to the friction circle of your tires. That is a HUGE concept in performance driving. The friction circle is this:
Imagine an x-y plot that goes from zero g to 2.0g. If your tires produce 1g there will be a circle with radius 1 from the origin of the circle. On the x axis we have our lateral g-forces (grip while turning left or right) and on the y axis we have our acceleration (usually -y axis) and braking (usually +y) forces. Now assume your car can actually produce 1g in each direction. Let us ignore the merits of that assumption for right now.
If we are in a purely right hand turn at max grip the graph would have a dot on the x-axis on the +x side at 1g. If we suddenly need to tighten the circle more and we are already at max grip in cornering what happens when we turn the wheel? One end or both ends of the car will lose grip entirely and the car will slide. Now what happens if we want to accelerate while turning? In order for us to accelerate while turning without losing grip we must reduce our lateral grip while we add power. In other words we have to straighten the steering wheel while we add power or else we will overload a set of tires and they wont produce grip anymore.
That is true for every car and that is how physics works. Here is what that friction circle actually looks like:
