the folks who work down at KRVS can chime in on this one.
when I was a line guy there I refueled a G-IIB for L-3 communications, its the one with the giant fuselage modifcations on top for camera sensors. They did military flights in front of VIP aircraft such as Air Force One, etc. for missile detection...and such.
anyway, talking with one of the pilots (they were all cool guys over there) he said that they tend to get up to FL550:rawk: on some of their missions, but are consistently above FL500. who knows how true though:dunno: