Just my two cents, but as someone who takes that flight reasonably often and usually comes by train, that would be way too close for comfort, at least for me. As phlwookie has already signaled, boarding is variable and while it is usually a bit late, it can be early and it can go fast.

For me, however, the big problem is that that airport train is not reliable. When it works, which is most of the time, it's great. But most of the time isn't all of the time. The train actually originates in the northern suburbs, passes through various junctions, and can get late. Amtrak screw-ups can play havoc with the switch-off south of 30th Street to the airport. (I'm not trying to be anti-SEPTA or anti-train here -- I use them all the time -- it's just that I'm pro-CDG [the airport as entry to the city of Paris, not the dead French president] and would hate to miss a day in Paris due to SEPTA.)

Or due to PHL airport and the TSA ... I find line-lengths at A at this time of day absolutely inscrutable. Usually fine, occasionally mind-numbingly slow. And the adaptation of the various entries to TSA Pre-check has been phlegmatic.