If I were the heads of EU countries, and this actually happened (after the already huge refugee crisis happened and keeps on happening), I would be <i>extremely</i> pissed off at the US for creating this problem for Europe in the first place - if not in public, at least in backrooms.
I'm not sure anybody has much incentive to report this number accurately. Whether it's ISIS, European security officials, or the AP itself, it's in the best interest of all of them to report the highest estimate they can find.
I guess I can't get too excited - I don't know what the ratio of "terrorists trained and sent to X" to "terrorists that end up not doing anything" is. We know the latter exists: the November Paris attacks suspect Salah Abdeslam apparently changed his mind and didn't blow himself up (<a href="http://www.theguardian.com/world/2016/mar/19/paris-attacker-salah-abdeslam-was-supposed-to-blow-himself-up" rel="nofollow">http://www.theguardian.com/world/2016/mar/19/paris-attacker-...</a>). My guess is that there's a pretty high drop out rate. But we'll never know, as that kind of statistic is deeply classified.