The 50K psi only tells part of the story. The units are "pounds per square inch" so in order to be meaningful you also need to factor in the square inches. "Bolt thrust" is the force applied to the breech face at a given pressure. The below is oversimplified but it illustrates the point:
.30 Super Carry = 0.0765 square inches x 50K = 3,823 lbs.
9mm Luger = 0.0990 square inches x 35K = 3,464 lbs.
9mm Luger +P = 0.0990 square inches x 38.5K = 3,811 lbs.
Sooo, although the Super Carry 'might' have 15K psi (43%) more than a 9mm, due to the reduced area of the case head the bolt thrust is only about 10% greater, and basically the same as a 9mm +P. Other factors, such as hoop stress, are similarly reduced due to the .30 Super Carry's smaller diameter.
In a similar handgun, the .30 Super Carry barrel weight is increased due to the smaller diameter bore. With the bolt thrust so close the handgun designers will hardly bat an eye.
Not much difference there in your calculations. Another thing I noted in my QL simulation, 50K was attained with only 5.9 grains of powder. So if that's somewhere in the ballpark, the overall gas volume will not be excessive, even though bore volume is less than 9mm. So it would seem reasonable that pressure will in fact peak and start dropping fairly quick.
