Simpson is an Indian company that has been making Perkins engines under license for many decades. The Simpson engines are Perkins designs but not Perkins manufactured.
I recently bought a 2635, which also has a Simpson engine. It's far too early to provide any useful feedback but I bought it for much the same reason that you are considering the 2615.
I wanted a basic, no-frills, 50-60 HP, open station tractor for mowing duty. NH did not offer quite what I was looking for so I shopped Kubota. The incentives that MF were offering allowed me to buy the 2635 highly optioned and with FEL for almost $5,000 less than a similarly equipped Kubota M7040. It's also nearly 1,000 lbs. heavier than the Kubota and rated PTO speed is only 1,790 engine RPM, which is a major advantage for my purposes. I do NOT like running my engines at 2,600-2,700 RPM in order to achieve 540 PTO RPM for mowing. Doing so wastes fuel and and racks up hours. The design of the MF allows me to cruise along at 1,750 RPM or so with plenty of governor and foot throttle left for the tough spots.
Note: The 2600 series is built in Brazil. There are known quality issues with the Brazilian plant.