The 2004 model 8 that I recently purchased used was running RX7 plugs -NGK BUR 9EQ in the trailing position without use of shims. These plugs are about 3mm longer than oem plugs so hence need for the shims..

What damage if any could this have caused to my engine ?

Read somewhere that the longer RX7 plug presses against the engine housing and can cause it to deform over several heating cycles leading to low compression in the engine.

What do you guys think ?

 

Not sure why the owner would possibly do that to the engine, regardless of what effect it has. The series 2 ignition system is very strong, and would be greatly reduced by old RX7 plugs.

I would get that replaced immediately.