to echo above the first thing you're gonna want is a chrono if you're going to be messing around with springs etc.
not only can it check the power is correct, but it's a good measurement for consistency and wether or not any remedial work done to the air seal has actually helped.
on the subject of which for range/accuracy you're gonna want to look at the following (in order of difficulty-performance ratio):
1. clean the barrel, and get into the habit of cleaning it regularly. a good rule of thumb is before every game day although sometimes even during a game day might be necessary (especially with a tighter bore)
2. feed it good quality heavier ammo, even the best gun in the world will shoot like shit if you stuff some bulldog 0.12's in it. how heavy is a toss up between your usual round count and your bank balance but generally the 0.28-0.32g range tends to be a good compromise for assault style play.
3. tying into 2 is to look at the hop, doesn't need to be difficult just a drop-in setup like the maple leaf macaron+omega nub combo when installed properly will help spin up the heavies. the longer feedlips can occasionally cause feeding issues in some guns and for those i'm a fan of the pdi w-hop with standard nub (although that'll struggle with really heavy ammo).
4. consistency- you'll need a chrono for this. you want to be addressing any potential leak so that every last molecule in the cylinder is going nowhere but down the barrel propelling the bb. the way to check this is to chrono a few shots and semi and see what sort of range of values you get, the smaller this range the better. this involves cracking open the gearbox too so a big step up in the difficulty.
5. energy- this is where you've jumped ahead to already, but the reason to wait until you're sure the air seal is good is you want to be sure the spring your using is only as strong as it needs to be to get the energy you need. if you have a bad leak and compensate with an overpowered spring you're just loading the box unnecessarily and with less resistance the piston slapping home isn't going to be happy. you don't need to be too exact, being a few fps under the limit won't be a massive loss in range but it might make the difference if the battered to crap filled with shattered nuprol bb's and soaked in mildew from sitting in a shed site chrono doesn't read so precise any more.
6. barrel- at this point the rest of the system should be getting good enough to justify a better barrel, gains aren't likely to be massive here (we're getting to the steep bit of the diminishing returns curve) but something like the ZCI 6.02 stainless barrels are cracking value for money. length wise just stick with the same length as stock, extending the barrel length won't help performance and indeed can actually make things worse if it starts under-voluming the system.
worth noting whilst i'm using the conventional "fps" terminology, that's actually "fps equivalent for a 0.2g bb", when using heavier ammo the raw fps number the chrono spits out will be lower, but don't be fooled into thinking that means less range, the energy (aka joules) is what matters (for site compliance) and even if it's travelling slower heavier ammo at the same energy does travel further (among other advantages).
as for semi-auto response, these days that's easier than ever thanks to the abundance of mosfets with precocking. that'll take a lot of the perceived delay out of the equation without messing with rate of fire in auto (and the potential problems driving a gearbox too fast can have). pair that with a decent motor (warhead brushless being the current kings for pickup speed, although they're not exactly cheap) and you're pretty much there already.
