We booked ComfortFlex for a RT ATH - JTR flight. That gives us 8 kg max for our carryon weight. If we upgrade to business class, the weight limit goes up to 13 kg for the carryon.

BUT upgrading to business class is $216/person and our flight was only $167 including Fast Track Security.

I am sure our carryons will be more than 8 kg. This is part of a 15-day, 3-country trip for which we are not checking bags at all for any of our flights so we will be cramming as much as possible into our carryons.

1. Is it worth upgrading to business class for the extra 5 kg? How strict are they with the weight? If we're over 13 kg anyway we will have paid the extra $216 for nothing
2. What is the Business Class Upgrade Challenge? My booking receipt says "Bid for an Upgrade to Business Class". Does that mean we might get Business Class for less than the $216/person?
3. How awful is checking a bag if we have to go that route? Will it take forever to get the bags in Santorini and especially in Athens? We really don't want to ever check a bag in case of loss.

Thank you for advice!