AMS is huge, but I don't think it's a major consideration. Getting from A to B is totally doable in 2 hours. Immigration and security (both in and back out) are reliably efficient and unless during a school holiday, wouldn't require much time to budget. The real issue becomes possible flight delay and wait time for bags. I wouldn't say 50 minutes for bags us usual, but it does happen at times...and I almost always wait at least 20 minutes. Even in J (where you'll get through the bag-check quicker) I don't think I'd risk the 14:05 arrival if no other reasonable contingency should you miss it.

As for things to do to kill 6 ours in AMS, there are plenty of shops, bars and restaurants and most everything is open during the peak hours you'll be there. The Aspire lounge is open, but BA have thus far not opened their lounge. You won't want for places to relax though.