Since you're boosted and probably already running the proper timing for your fuel you probably won't benefit much from anymore advance but you will see longer life with them and less high rpm misfires. It might not be something you can feel in the seat but it would be shown on a dyno.
Use the m50a motorcraft spec ngk plugs. They come gapped at .030 so closing all the way down to .022 if needed is acceptable they also come with an electrode arm designed for boost that is much stiffer then the 6510. The reason you're seeing inconsistent gapping from the 6510 is because of the...
The 6510 is the stock heat range for the gen 3, it is one step colder though for a gen 2 car like mentioned. Either way a 6510 should not be used for boosted combos because out of the box they come at .044 gap and ngk never recommends closing the gap more then .008 it causes the electrodes to...
Like already mentioned make sure no pipes are even close to touching and yes the smalll filter is a restriction but if you only plan around 600 rwhp then it's ok.
I've literally been saying this for years the ngk 6510 is not a boosted sparkplug, yes it does work but was never intended for boost. It's not made to be gapped less then .036. The electrodes are no longer parallel which causes other issues and accelerated wear and misfires. Any tuner still...