When you raise the car, you are also raising the centre of gravity. It is a known fact that cars with higher centre of gravity don't handle as well as cars that have a lower centre of gravity. That is why sports cars are designed with low ground clearance, to make them handle better.
Since your car is only raised from the back. it creates a new problem. Not only are you raising the centre of gravity, you are also creating an imbalance in the weight distribution between the front and the rear of the car. It causes the weight to shift forward on the front tires. This is especially a problem when you are braking hard. While braking the weight will shift forward even more and the rear tires can lose their traction. This can cause your car to fishtail in a corner.
Coming to your question as to why you are experiencing less grip now and not 5 months back. The reason could be that either your tires have very little tread on them or the rubber has hardened which will also cause the car to slip. Since your car is rear wheel drive, hard tires combined with less weight on the back can cause the rear wheels to spin when you accelerate.
The first thing you should do is thoroughly examine the tires to make sure you have enough tread and the rubber is not hard. Your problem could well be more because of tires rather than ride height. But in any case you should still lower the car back to stock.
I hope that helps