In my opinion, 225 street tires would be way to wide for the stock 17x7 wheels. They'd be very sloppy.
205/50-17s would increase your outer diameter. The additional sidewall would give you more cushion, but it would hurt your acceleration and unsprung weight.
205/50-16 is a stock tire size, and the OD is very close to the 205/45-17s you have now, so the speedo won't change much. You can do the calculation yourself if you know how to read tire sizes.
I think Brian's suggestion of the stock 16s was aimed principally at adding more sidewall. The stock 16-inch wheel/tire setup is the same weight as the stock 17-inch combination, so you won't save any unsprung weight going to that.
Lower unsprung weight always helps the suspension respond, so it helps both ride and handling. I'm not sure if you will get enough benefit to warrant the expense, though. That said, there are some very light 16x6.5 and 16x7 wheels out there, and Brian offers a number of 17x7 and 17x7.5 wheels that will save you 2-3 pounds per corner. Example:http://www.good-win-racing.com/Mazda-Pe ... 290NC.html
I still say you should start with a good alignment.
2009, STR & DD. 1995, HPDE. 2004 MSM, sold. 2010 Mazda3, hers.