We owned a Catalina 25 with a dual axle, Trail Rite trailer for many years. Then we built a triple axle trailer for a Catalina 28 and now have a Catalina 30 on it which weighs at least 10,000 pounds. I towed it 1,200 miles last summer and it was solid, safe but still a load. I figure the trailer and boat were at leat 12,000 pounds. By the way, a 3/4 ton Ford diesel towed it just fine.
The difference is three 5,000 lb axles to carry the trailer frame itself plus the boat and stuff. The next difference is the frame. Our Trail Rite was box iron (open on one side). The other trailer has a full box frame easily 2" x 6" (maybe more, sorry, I don't know my steel). The size and weight goes up substantially from 25 to 30. Finally, the trailer must be much longer to fit the 30 foot boat because it needs more support pads. So, I'd think your plan would be safe going up a few feet from 25. Beyond that maybe you could sell the smaller trailer and invest in an oversized unit that allows you to pull the rig stress free? Good luck, be safe.