years ago I had a similar issue with a Yamaha sport bike. Turned out the problem wasn't in the alternator, or the regulator, there was corrosion in the main path between the battery and the ignition switch that was causing a pretty large voltage drop. The theory of operation of charging systems isn't generally spelled out in the service manuals, but the basic idea is the charging system will monitor the system voltage downstream from the ignition switch and try to keep that voltage within a particular range. If there's an unusual voltage drop in that path, the monitored voltage level will be lower than normal, and the alternator will increase the charging voltage upstream to whatever level is necessary to bring the monitored voltage back into the desired range.
For example, let's say a normal voltage drop from the battery to the monitor point is 0.7v, and the charging system wants to see 13.5v at the monitor point, that means the battery voltage would be 14.2v. If the corrosion increases that voltage drop to 1.5v, now the charging system will put out 15.0v at the battery to keep the monitor point at 13.5v.
I've never had to trace this out on the ST1100, so I can't give specifics, but if you look at the wiring diagram and find a convenient place downstream of the ignition switch to measure the voltage you can see what the voltage drop is from the battery to that point. If the drop is more than 0.7-1.0v, it would be worth looking at the connectors in the path for possible corrosion.