There is much more to worry about than the durability of the rods and pistons. The torque and power curves suggest there is more power to be had beyond the redline. So, one must wonder why Triumph didn't set it higher. One consideration would be valve float. There is a limit to how strong the valve springs can be when the cams are running directly on the aluminum head and caps. If you want to make serious increases in the rev limit, you would also need to line bore the heads and cam caps, and install some substantial bearings.
Another consideration these days is noise. The faster you run an angine, the more noise it makes. This is such a touchy subject that the Lycoming O320 installed in an airplane I once flew had an orange patch on the tach. You could run the engine above or below the orange patch, but it was illegal to hold it in that zone because the exhaust system and prop resonated there, making excessive noise.
Other things to consider are the cam drive chain and tensioner design, all that somewhat delicate looking machinery in the primary case, the effects of turning the balance shafts faster, etc.
