Wuerstchenhund,
the non-working signalling tests under DOS are not a limit of the license, but are caused by the CRTU-USU not being supported (at least in 4.35; I haven't tried anything newer) as a link handler. The "2G_Signalling" feature (that enables the signalling tests) requires CMU-B21Var02 or CMU-B54Var14 to be present; the CRTU-B5* is not sufficient.
Or have you ever seen a CRTU with the signalling features enabled? If so, I'd be curious about the hardware/software configuration.
And yes, DSB+FM was added in 4.x; it's unrelated to the IQ board. However, the reason why most of the non-B7 equipped CRTUs ship with V3.23 is that the Windows-based software isn't compatible with DOS OPSW > 4.0. While it doesn't use any of the DOS components, it relies on the firmware of the various boards having a specific version. (The firmware is updated together with the DOS OPSW; i.e. if you switch from V3.23 to something else, the board firmwares get updated to a version that's incompatible with the signalling tests in windows).
Now, the devices with B7 are mostly used together with a CRTU-PU (WCMDA); in that configuration, the CRTU-RU actually runs the DOS OPSW (which does little more than just setting RX and TX frequency, and configure the baseband/IF paths); all the Windows-based software runs on the -PU. In that case, the DOS OPSW on the -RU can be a newer version.
So - that's the reason why B7-equipped devices usually come with a more recent firmware than the non-B7 equipped devices. (Of course there may be configurations where you have both the USUs and B7, for example for fading simulation, but the majority of the CRTU-RUs come with _either_ B7 _or_ the link handlers.)