Will getting iridium sark plugs make a difference? if so how much?

Oh god yes. I used to deliver pizza's running 800+km per week so i upgraded te iridium. my car always starts, idles well, good power and helped with economy. saved ten bucks a week in gas. but remember that was about 800-900on a low week. I leave it sitting and as long as it gas charged battery and gas she fires.quicker than my daily too.

No, not a chance of it :p Any new plugs will be better than old plugs. Some plugs will last longer before they errode, but if you change them as per the Volvo service interval, it is just two plates of metal a set distance apart. Making sure all the volts get to the 2 plates is the key thing, what they are made out of will only effect how long they stay as two clean plates of metal at the same set distance.

Given how LE jet runs on 95% of cars I see, they'll all be sooted up and it'll be irrelevant within a week

Tbf, it's all personal opinion. Some people swear by them, I just can't see any reason in science to spend the extra.

Which is fair, i spent the extra and haven't regretted it.

I've always liked the Bosch super 4, nice and revy, never had the need to change them once fitted. Tried platinum point ones in 1.4 but engine didn't like them, felt held back.