For your uses, especially mowing 15 acres, the CK3510 makes sense. I jumped to a DK40se from a CK20 when my mowing needs went from 4 to 15 acres. However, it isn't correct to say you got 50% more tractor for 10% extra. What you got was more PTO power. The frame size, 3PT capacity and loader specs are IIRC identical between the 2610 and 3510.
If you need to run six foot implements then 35 hp makes sense but for 5-8 acres or so 5 ft implements are fine, cost less, are easier to maneuver and store and are only a bit less efficient. The 20% increase in cutting width can add up when mowing large tracts but in reality cuts maybe 15 minutes off of a 2-2.5 hour mowing job given transport and set up time. A wider mower helps but it isn't dramatic until you get into batwings Etc.
Mowing (or snowblowing) are about the only task where more PTO power is especially useful so if other tasks are stressing the CK2610, it would make more sense to move to the NX4510. I'd personally always want a lower available hp engine in a larger frame size than the highest horsepower in a smaller size tractor.