Depends on the gun.
Buckings are an easy part to get wrong. There's a lot of branded crap out there that isn't fit for the job. And since we tend to play in cooler weather over here our buckings need to be soft.
50 degrees or less is perfect for the uk@350fps. Soft is better here. So the likes of the madbull buckings that are all 60 degree will just give you crap performance. If you lived in a warmer location (chances are your youtubers do) then they work perfectly fine.
Same for inner barrels. The stock 6.08 china barrels might be mass produced and not the highest quality, but that coated barrel you are looking at is only as good as the coating. Give it 12 months and you'll be buying again.
The people that mess with gear ratios and motors. 18:1 with a fast motor is more than enough for most players. Total outlay probably less than £50. To give 25rps.
Doing the gears to a lower ratio, HT motor etc you can easily spend over £100. And still only be getting 25rps. So what have you gained other than a lighter wallet... The reason people do this, American FPS and springs, they have failed to realise our location (UK) has a different set of rules, and we can run the same RPS for far less investment.
The real trick people miss is a basic inspection. They look at the big parts, Gears, bearings, Piston, Heads. But fail to look at the important seals, and fail to understand how those seals work. A chipped or worn O-ring is only good for the bin, but if you can't spot wear then you are never going to get the performance you want. O-rings are cheap. Running at pennies each, Not replacing them at every opportunity is probably the biggest mistake people make overall. If you open the gearbox and it's been shot, spend the pennies and have new seals. You can buy 100 for £5.
Upgrades are hard to flat out condemn, or flat out recommend. There has to be a process to what you are doing with the upgrades and a reason to buy them in the first place. If your gun doesn't shoot straight chances are it's not poor components, it's poor fitting that has caused the issue. People are too willing to believe the gumph that retailers spew about how good internal parts are. There is no single part that will give you accuracy range and consistency. The only way to get that is to have all parts working well together.