I would agree with the comments made so far. I would go very happily for a 1 hour connection, something I do a lot, and it's been over a year since I've missed a connection at T5 (pre-notified delays excepting). I do go landside to avoid North Security, more often than not, however, and I can use the e-gates. I'd probably check the later flight to see if they were still selling tickets for it, and for a short hop to ARN I'd not get terribly stressed if I ended up in a middle seat, the flight isn't that long.

But the other factor for me is "what would I do when I get to ARN"? If I had a hotel room already lined up and I could go there and relax first then I'd want to be there as quickly as possible. If I'd simply arrive very early for a meeting then I wouldn't see the point of rushing and missing breakfast.