I'm with you on the understeer issue.
There are a few things to try first that are free.
First, front tire pressure is really important. You don't want the tire to be rolling over when you turn. Before buying anything, add pressure to the front tires and see if that helps. Too much pressure will start increasing understeer again, but often you need more pressure in the fronts than rears to keep the tire planted across the tread width. You can start with increasing it to 5 pounds over the factory suggestion. I move in two pound increments up or down from there until I know which direction I want to go.
You can also play with rear tire pressure the same way. I spent last summer running 37 psi front and rear, but 38 psi made the car skittish if the corner had any bumps or divots - turned quick, but felt like you had little connection to the pavement unless it was perfect. Ultimately I took it back down to 35 front, 32 rear because it almost felt like I was riding on basketballs
Hard to believe that a single psi made that much difference, but honestly it did. However, at some point you tire of splitting hairs and pick a comfortable pressure and call it a day for daily driving. But if your wider rear tires are doing their job but overpowering the front tires, you could benefit from technically too much pressure in them until you can address the issue with a sway bar.
Second, make sure you are using the brake prior to turn-in. A quick stab is all that is required to transfer the weight to the front and make the tires bite. It's really easy to do it right, just a quick stab of the brakes and turn the wheel and the front will either hook and yank the front that way or you'll hear the tires squeal as they roll over/slip. ~35 mph is a good test speed. Done properly, you don't lose much speed but the traction difference is huge. Apologies if you already do this. It took me a while to figure it out for myself.
I actually want to feel the
suspension compress when I do this as it means the tires are grabbing.
You should be able to get on the gas pretty quick without losing traction (not slamming the gas to the floor, just pouring on the power). If the front starts to slip when you apply the gas, you definitely want a stiffer rear bar.
You want an adjustable rear sway bar. The advantage is you can tune it to what you need rather than just bolting on a piece and hoping it makes things better. A number of companies make adjustable bars, you just want on with more adjustability rather than less. The Steeda one looks better than the H&R piece in that respect, for example, but ideally someone who has tried each can offer input.
I use a cloverleaf near here for higher speed "testing," as it is consistent and safe during low traffic periods. I use city streets and empty parking lots for lower speed testing. You can learn quite a lot without breaking the speed limit. Quite honestly, the stock configuration is quite good with proper techique. To really make it better you need to pick your mods with care rather than just buying a package from a vendor and calling it a day.
Just like drag racing, better tires are the best performance mod. But if you can get the car to drive the way you like, better tires just make it perform better. Putting stickier tires on a sketchy
suspension won't fix anything. You'll appreciate the better tires more when the car's behavior is sorted out to your liking.
I'll bet mine is worse...I'm sitting on a bone stock, 100k mile worn out 
