Prius not running right, Please help...

Hello, I recently had my catalytic converter stolen on my 2006 Toyota Prius (234,000 miles). I also installed a cat shield myself about a week after I had the cat replaced at my local Toyota dealership. After I installed the cat shield myself, I did notice rattling, however as the cat shield is simply an aluminum shield covering my catalytic converter, I do not believe it is the cause of my problems. Recently I have noticed the mpg on my Prius deteriorate noticeably, I was averaging about 45mpg around town and now I am doing about 35 mpg. I notice the engine having to turn on a lot more often than it used to when idling the idle sounds quite rough. As well, I have noticed that the acceleration is extremely constrained, almost like the emergency brake is still on. I takes much more work from the engine to push up to highway speeds which I feel is why the gas mileage is dropping so much. I do not believe the spark plugs have been replaced anytime recently, at least not since I have owned it (almost a year). Any advice would be greatly appreciated.