I spent 7 years in direct maintenance support working on machines from all over the world. I have been working in machine build for over 16 years with the last 6 years being clean sheet custom build. Rarely do we build the same machine twice. We also often support machines from other builders that will either no longer support their machine or the customer won't let them back in the door.
Having a higher switching frequency helps in reducing size of components such as transformers and inductors and help achieve higher power density. A smaller size will typically also mean lower cost of the product and a smaller physical size of the product.
The difference is the soft starter is primarily a mechanical soft start. You reduce the voltage and limit the current to ease the mechanical stresses associated with motor starting. You do have the potential to reduce the current associated with starting, but this benefit is highly dependent on many variable and can change dependent on the particular voltage condition at the time of starting. Medium Voltage Motors do not have the generous amounts of starting torque many people take for granted with 460v motors.
The way a soft starter works is NOT AT ALL like an inverter. All it is doing is retarding the gate (turn-on) time of the SCR so that only a portion of the AC sine wave gets through. Inverters create pseudo sine waves by PWM firing of DC with high speed transistors. Totally different and there are no "pulses" with Soft Starting, so there are no additional shaft currents created. The soft starter is NOT the cause of your fluting.
The capacitors may improve line voltage but not likely affect your metered load unless you are suffering from sharp power factor. penalties. If you have .96 pf now I doubt the penalty is too steep. Need to run a cost benefit analysis for some limited losses savings but I believe the changes due to a bolted start will cause greater impact than you might experience with soft starter. Check with your utility as well. They may have sized their circuit based on your soft start.
The refinery I am working is fairly optimized in the energy department, so most of the transversal measures were already implemented. We already have capacitors in the general low tension switchboard, there are variable frequency drives or soft starters in almost every major motor, the lighting system was already retrofitted.