My opinion would be to go back to standard plugs. This way you know you are using the stock item - recommended part.
Firstly, If you suspect the exhaust take it to any local tyre/exhaust place and get them to check it. It shouldn't cost you any thing for a check up. Or let the car idle and before the exhaust gets too hot, reach under the car and feel for it blowing around the centre section.
I suspect if you rev the engine with the car stationary, you will not hear it blowing over the sound of your back box (unless it is really bad!!!).
To be honest, upgrading spark plugs on a standard(ish) car will make no difference.
Check the exhaust first then go back to stock plugs and if you still feel it's lacking you will have to have a diagnostics or trip to a RR to check your map/fueling/timing etc.