[Red Mustang Gt 46]

Here goes...my recent experience.
I replaced my stock plugs at 50,000 miles with Bosch Platinum +4's.....My car had a VERY hard miss from idle to 2500 rpm's after the install.

A month later, I replaced the +4's with NGK's, what a world of difference. Now my car purrrs smoother than my Crown Vic..it is genuinely scary.

NGK has discontinued the TR55VX's and replaced them with the TR55IX (iridium). You can still get what's left of the VX's and the IX's are $7.59 at NAPA auto parts.

As far as performance, slight gain (sotp) but the gas mileage went from 250 miles a tank to about 300 miles. It was worth it for me, I can't get over how well the car runs now...better than new.

Good luck on your decision...