I am a lot more familiar with conventional keelboat rigging so I am sure others can add to or refute what I have to say, but it is my understanding that on a rotating mast the shrouds are not functioning to shape the mast but only to hold it up. The diamond stays along with downhaul and mainsheet will control mast shape/sail shape. So on the catamaran with rotating mast your shrouds need to be tight enough to keep the mast from falling off/flopping around and loose enough so that the mast can still rotate. So really shroud adjustment I would suppose is just a matter of how long you have your forestay set, but forestay tension is still controlled by the mainsheet. It sounds like you have yours about right. The only reasons I know to adjust them is that it can be advantageous in light air to have slightly looser shrouds so that downwind your mast can rake further forward or in a capsize if you extend a shroud on the side out of the water it can make recovery easier.
Actually, the luff cut/curve along with the downhaul puts a shape on the mast; the more you yank on the downhaul, the more the sail curves the mast. (If you roll out your main, and tack it down, you can see curve on the front of the sail. If that curve is not there, your sail needs the luff rope (bolt rope) to be released -they shrink A LOT over time!) By comparison, Tension on the mainstays, facilitates adding a little more curve, but mostly positions the rake (how far back) the mast ‘tips’. In lighter air looser shrouds allow the mast, and sail, and therefore ‘center of effort of the rig to pitch forward. This provides a bit more power (esp downwind), but in heavy air, it can work against you by allowing the main to leverage the boat to far forward and pointing your bows ‘down the mine’. Most cat sailors, rake it back in heavy air, depowering the rig. The more aft the main is, the better it points, too.
