98vs100 gasoline

If the soft is made to run 95-98(europe) gasoline and I run 100, does it affect the EGT? Is the EGT higher?

I may stand to be corrected, but the higher octane should burn slightly slower, so may have slightly more energy passed through to the exhaust - but I very much doubt it will be of any significance.

What is the engine it is being used in?

1.8t 20v

You might find that EGT is actually lower as the higher octane fuel is more knock resistant meaning more ignition advance can be used therefore the peak cylinder pressure happens earlier in the expansion stroke leading to less pressure ergo temperature when the exhaust valve opens.

I think he means it was tuned for 98 and is concerned there might be an issue with running 100 on that tune - can't see it being anywhere near being a significant issue.