What is the maximum speed for regulation of sonic boom noise?
Business jets are developed to different rules than airliners. The biggest difference is that, indeed, fuel price and fuel efficiency are not an issue when what counts are bragging rights for the fastest plane on the ramp. Their speed has steadily increased and is now at Mach 0.935 for the fastest models. Compare that to Mach 0.78 to 0.85 for regular airliners.
Supersonic business jets were proposed several times in the last decades, but so far nobody has risked to spend the money it needs to bring one design to a type certificate.
Since supersonic flight over land is heavily restricted, the fastest speed would be Mach 0.99. However, since the speed of sound decreases with increasing altitude in parallel with air temperature, what is Mach 1.15 in 40.000 ft becomes only Mach 0.998 on the ground, so the shock will disappear close to the ground. If the goal is to avoid the shock on the ground, flight speed high up can very well be mildly supersonic. However, this will place the airplane close to its drag maximum, giving it exquisitely poor efficiency for a small gain in speed.