I don't know all the in's and out's, and have only tried a few things. But this is my basic understanding:
The first thing to consider is that most airguns are now designed for overseas markets, i.e without the 12 ft-lb muzzle energy we have. In this country we get the same basic high powered design, but throttled back in some way to comply with the law. For example soft springs (TX200) or transfer port restrictors (Diana springers).
This means we are generally getting an inefficient design.
The general idea of tuning is to either increase the efficiency of the powerplant, or to make the shot cycle feel how you like it. Which route is best is a very individual thing.
The basic drop in kits will use the standard piston but replace the metal top hat with a plastic one. The metal top hat helps to achieve power on the overseas guns, but we don't need it so the weight comes out. This means less mass is flying around inside the gun and therefore recoil feels less.
Short stroking, will decrease the volume of air available to the piston. As before, high powered guns need a lot of air to make the power. As we can't have access to that power, we are compressing the extra air for no benefit. So reducing the available air to just the right amount, can increase the overall efficiency.
Reducing piston weight again means less weight flying around, but needs more spring load to achieve the power you want unless you can balance the spring/transfer port/pellet selection. At this point you are getting into the realms of tuning the gun for specific pellet (due to pellet start pressure/piston slam/piston bounce)
Then reduced diameter pistons are again trying to increase efficiency and use lighter weight components. Due to the much higher pressures generated, pellet and seal selection seems to be important. Some people have reported more hold sensitivity but I can't say for sure.
But all of this means nothing until you pick the rifle up and try it. I've had a go with most tunes in other peoples rifles, and actually prefer a more basic setup for HFT.
Some people prefer quite harsh shot cycles that feel 'fast'. Other's prefer a softer cycle that feels 'slow and lazy'. I'm somewhere in the middle.
I actually think a HW77 is almost perfect as it is, so you have a good starting point.
And as always, no amount of tuning will buy you points. You still have to put in just as much practice and work as anyone else. But what you can do is make the rifle more enjoyable to shoot. This is where I think the main benefit lies. If it's fun to shoot, you will shoot it more often and be more relaxed doing so. If you don't like how it shoots, you will lose focus and won't want to practice, regardless of how fancy the tune is.
Hopefully someone more informed will be along shortly to correct everything I've said