It will make a difference, but it needs to be done with other modifications (porting, cams, full exhaust system) to really get the best out of the engine.
I have high compression pistons (but not the other mods) and I can't definitively say it was worth the expense or effort, and now my VTR won't run on regular gas.
Installing the pistons was something I was determined to do; nothing would have dissuaded me. Eventually, I will do cams and exhaust.
Shortening the gearing gave a much more noticeable kick in the pants for a whole lot less time and money.

 

My bike has Moriwaki pistons and cams from a kit off of e-Bay. Engine performance is always a balancing act, you have to re-jet the carbs to get the potential from the cams and pistons and at the back end, you need free flowing pipes in order not to constipate your effort upstream.....then there's the question of the stock frame and suspension being able to handle the enhancements.

The dyno said 65 ft lbs and 126 hp on hi-test. Because of the PITA factor, I think I would have gone to a RVT and just been done with it in retrospect.

 

126...wow, I don't think I've ever seen a VTR that high

 

there's a guy in italy that's up there with that number