This subject is a vast quagmire.....Really difficult to discuss in generalities.
Some ballast is common in production trawler type yachts. Generally the lower speed vessels will have more and higher speed (semi-displacement hulls) will have little or none. Again a really general statement would be that; "heavier displacement (slower with larger ballast amounts) boats will have a lower center of gravity, thus higher angles of vanishing stability (AVS), than un-ballasted semi-displacement boats."
Many boats of any type will have trim ballast here and there. Larger volume hulls (wider and deeper) will often carry large amounts of ballast, typically converted fishing boat hulls.
The amount of ballast can vary wildly and by itself says nothing about the boat. To my mind, if anything more than 10-12% of displacement weight is ballast, you're hauling too much dead weight ($ down the drain). More typical pleasure boat numbers would be 3-6% of displacement.
"Although not pitched as a bluewater passagemaker, the SP Cruiser has a reassuring stability index of 39 and a capsize righting moment of 155 degrees"
It depends on your definition of reassuring. A Catagory "A" (Unlimited Offshore) rating (STIX above 32) places operational limits at "Significant wave height of 7 metres and wind force at up to Beaufort 10." Beaufort 10 is a whole gale, 48-55 knots of wind speed.
The stability index (STIX) number really is not applicable to a powerboat, it's a comparison number for sailboats. Your power version of this sailboat/motorsailer is modified with removal of the rig but no (?) reduction of ballast weight. Therefore those numbers (from the sailing version) are meaningless. Ask the manufacturer for new ones.
The second part of that statement makes no sense. A moment is a force, not an angle. It could be that they are trying to say she has an angle of vanishing stability (AVS) of 155 degrees. That's quite high, undoubtedly due to the volume of her big high deckhouse. But I bet it does not account for the aft and mid cockpit being full of water.
Also the A STIX number allows a minimum downflooding angle of 30 degrees. Meaning that water starts filling the boat (downflooding) at something over 30 degrees heel, making the 155 degrees AVS utterly silly. But it's good marketing......