I've run Denso and NGK on E85 for 10k+ miles and numerous track days, with zero issues.
Holy shit - $75 spark plugs? I think my Honda plugs cost like $5 a pop for NGK iridium
I replaced everything with Motorcraft plugs and wires and it has run great since.
Also, you should probably go with NGK spark plugs because if you put Bosch ones that are made in China/Russia you are going to have a baaaad time
My RX8 also got pretty piss poor compression numbers. The only time it struggled with a start up was with literally the original 20 years old plugs it came with and I had to do multiple hot starts. It still started and carried on.
Currently will be doing all fluids, filters and spark plugs so the car is properly serviced.
They run just fine on stock setup. Changed mine 6 months ago once the car was at 30k miles and actually managed to snap one of the plugs in the head ???? Carefully managed to get it out though. No noticeable difference from factory plugs.
I have the ruthenium. Bought pregapped from Mountune. They are fine. They work no noticable change. I've put about 200k miles on them between by ST and RS and changed every 50k miles with no issues (both tuned and stock).
If it is the skyactive-x be aware that the owners manual advises spark plug replacement at 40k miles and the Mazda plugs are \u00a340 each. I put aftermarket Bosch plugs in and it ran very lumpy and had to replace with Mazda ones.
I changed my plugs for NGK IRs but now I have a rough idle when the engine warms up.
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