At BOS, you can definitely make a shuttle if you pull up to the airport door 15 minutes before flight time. Probably as close as 10 minutes before, in fact. The shuttle advertises that they hold open the aircraft door until 3 minutes before scheduled departure time. It is part of the product marketing, because people who value their time can differentiate between two hours door-to-door via the shuttle and four hours on a train or in a car.

I would not recommend arriving at 15 minutes on a regular basis. However, I think 25 is plenty. Passengers frequently determine which shuttle to take based on how quickly the taxi gets to the airport.

In DC, does the shuttle have its own security (TSA) line? It does in BOS, and this is crucial. If TSA is getting in the way of the airline's ability to market the shuttle to customers, then more letters are in order to Congress. I believe that the airlines should be served by TSA, and not vice versa. The airlines are the customers of TSA, just as the passengers are the customers of the airlines.