I wish subs had been the first mod I had done!
As said above, you'll notice better steering response and control in cornering. Go find your favorite curve and try it again with subs on. You should notice a difference.
For me, I noticed better control in turns and the car felt tighter overall. Given the age of our cars and the fact that I've got over 156,000 miles, it is nice that my car has no squeaks, rattles, or other odd noises due to age. There are many cars at my work that are the same age and newer than mine that squeak really bad.
With the setup and power you're making (from your sig), subs will save your car in the long run. You've got several suspension upgrades, so you may notice an even bigger difference with the new subs (then again, maybe not quite as much).
Let us know what you think.