Sparkplugs- NGK V power vs G power

Lately my truck's seemed to be a bit slower, less mpg and in looking at my records I realized it's been about 2 years since I changed my sparkplugs myself, but in aug last year on a long trip my truck had a melt down and I needed to get it fixed out of state. The shop changed the rad and cooling system but also put in a new #3 coil and apparently a new sparkplug in that single cylinder. I only found out just today when I changed out all my plugs and put in NGK v-power plugs. The #3 plug was an NGK platinum plug, with the gap at about .05 instead of the factory .031 gap. All the condenses down to a few questions:
1) can mixing plugs like this do anything harmful?
2) are the platinum plugs any better than the standard ones?
3) how much performance is lost with a larger plug gap?

 

Makes me think the shop just put one in to charge me more money. It definitely feels better with new plugs in. The shop also hadn't put any anti-seize on the plug, the 3 I had put in came out with no problem at all, the platinum plug took enough effort for me to think I was going to strip it or break something. Lesson: don't ever trust an unknown shop, if you have to check the work afterward.