Enjoy a reliable bike, you'll pay a lot each hp and they won't be that impressive.
XT is a torque mono, play with that and enjoy.
Power is the ability of the engine to remain on high rpm's as a supersport going upwards a hill without losing any rounds once in spin.
Torque is the raw strength of the engine, ever tried to screw a 22 nut with your fingers..take a long grip, that multiplicates the strength from your hands, torque is the same thing and is due to many things; size of the piston, weight of the crankshaft, weight of the rotor, gearbox demultiplication, clutch demultiplication etc.
Power and torque is a balance difficult to obtain by the yamaha enginers.
An XT is made very good orginally, the balance shaft compensates vibrations and has exactly the same weight as the original piston..torque is well distributed from 0- 5500rpm.
Changing the piston might give you naughty vibrations, only that makes the engine much less reliable.
You will change the engine character and in the end probably the bike will be less performant than original, high compression will create more heat to your aircooled cylinder, the vibes will torture your allready worn conrod and bearing + heat and torque on that= hello engine brake.
I won't say for sure, but I wouldn't do those changes, everyone is free of course.