If you're asking how to get better stability in general then thats a different question entirely.
Were you feeling squirrely in the straights, turns, or both?
Since you mentioned going 115mph I assumed you were going straight. In that case you need to create downforce or achieve a lower center of gravity. A wider stance would also help. To create more downforce you need a diffuser like you asked about. A diffuser can go many places, front lip, spoiler, top of your rear window, under the rear bumper, etc. Diffusers just create a low and high air flow channel around your car like an airplane wing.
You can get a lower center of gravity by lowering the car.
Finally, you could widen your stance by A. picking up some extended wheel studs, B. getting some wider wheels, and C. a lower offset. Any of the three or all combined will get you a wider stance increasing your stability.
If it's turning then like trentimus said, sway bar will help you out. Are you understeering or oversteering? Knowing that will tell you which sway bar you want to upgrade first and how thick you want to go. Replacing your stock bushing with newer better designed bushing will help out a lot as well. Lastly, you can lower your center of gravity which will be your best idea because if your center of gravity was low enough you wouldnt even need sway bars.
Just keep in mind sway bars will only really help in cornering. When you take a right turn the body of your car tends to sway to the outside of the turn.A sway bar ties one end of your car to the other. The more your car starts to sway, or roll, the more the sway bar pulls on the other end. When going straight, the sway bar doesnt do much. Chassis bracing will basically do the same while in a straight line as well as create more rigidity to the structure of your vehicle. Increasing the front sway bar will plant the front and let the rear sway more. This will allow your back end to fish tale slightly more when turning at high speed. Increasing your rear sway will keep the rear end tight behind you, but too much will start to affect the way your front reacts to a turn.
ALSO KEEP IN MIND SUBFRAME TEAR OUT.
As I mentioned, sway bars pull on one end of your car. The harder you turn, the harder it pulls. Sway bars attach to the subframe with two bolts. Cars have a tendency to rip out the sway bar if its not properly braced to the subframe by a... well, subframe brace. Long story short, the car body rolls like a fishing pole bends. A beefy sway bar is like a stiff pole and the bigger the fish the faster the turn. That stiff rod works great for small fish, but when a big fish catches the line itll probably break the line before the pole bends.
