I had similar issues, infact - still freakin do!
car was spluttering and extremely laggy on gear changes, had 7 runs at WSID and all did 14.7
got plugs changed, car felt so much stronger, no spluttrering or anything on gear changes or when backing off.
but now, car is running like shit again, a few ppl think maybe my car is running a bit too rich and fowling the plugs causing it.
so im not happy yet again.