Blackdog, many years ago (85-90) I had a Seca Turbo as a street bike. You are absolutely correct it will rpm limit out at high 120's mph, as verified on the way to Las Vegas from so-cal on I-15
The shaft drive prevents any easy gearing change I am aware of.
On mine, I removed and plugged the wastegate hose that senses boost, so in effect the wastegate sensed zero boost all the time. Stock boost is about 7.5 psi, this mod allowed it to get 14.5 psi
The 14.5 is limited by the pressure blow-off safety valve on the back of the carb bonnet. That became my boost limiter
Cheap and easy power increase, drove it this way for approx 20K miles. I did splice in a vacuum/boost gage so I know the values are accurate. Also spliced into factory oil cooler line and hooked up an oil pressure gage; mounted both gages in the top of the dash area right underneath the windscreen. Used Auto Meter 2 inch gages.
The basic problem with the bike is the poor low-end before boost starts to come up. The extra weight and low torque made it a dog to take off standing start. Once on the boost, it would pull like a friend's liter bike, but was a dog until 5000 rpms. Never had any reliability issues, it was a good bike for commuting and riding around - just not for drag racing or landspeed racing.