i barely noticed lag with my blitz fmic (but i did notice it regardless and didn't care because i always wanted a fmic when i had it done it was the 2nd mod i had done (from stock) at the time, but i was advised to get one due to i was upping the boost from stock and a fmic would be recommended to avoid risk of detonation and increase power gain etc .. (was told) i think its a must have if your planning on running more boost (regardless if turbo is stock) but that's my opinion meh
I had a dodgy 02 at one stage which made my car run rich as all hell , wont damage the car just might run like a dog and use a heap of fuel something you want to replace as soon as you can regardless though. If the plugs are that damaged even get a hold of some plugs from the wreckers and rewire it