To clarify, the spec was originally written for a heavily suppressed 9mm platform, the XM integrated does have mediocre velocity due to the series of vents/ports along the barrel in the integrated suppressor the design completely blocks all muzzle flash and almost all of the gases.
The actual unvented barrel length is about 5" in rough numbers.
1,100fps from a stock G34 with plain retail box Federal 147 is very good, the best I have clocked has been 1,030fps at what altitude are you getting that velocity?
The Federal 147gr +P+ averages 1,240fps from my G34.
I believe that the Glock rifling is able to increase velocities by as much as 10% depending on the chamber cut in your barrel, some are higher and some are lower.