I'd ask what you want to get out of the components and whether this represents good value (if so whether you care about this). You also want to look at the knock-on costs; for example if you have a 9sp groupset you may have to change your hub to fit an 11sp cassette; meaning realistically the rims are going too..
In terms of groupsets IMO 105 is the sweet spot - nice performance with massively diminishing gains / value if you go any further up the chain. That said Tiagra is nearly as good (with the exception of the brake calipers) if you're happy with 1 less cog on the back.
Wheels and tyres are the most effective way of managing performance through mass loss, but even then I'm dubious of the claims and insinuations made by the industry. Talking mass generally (not the rotating mass of the wheels) 1kg off an entry-level bike might reduce its mass by around 10%, but factor in the mass of the rider etc and the saving drops closer to 1%.
Think how much it costs to save a kilo on an entry level bike; just to get you up that hill / accelerate off the line 1% faster.
I think we all get carried away when considering upgrades; however in reality IMO most of the time the tangible benefits are minimal (certainly beyond a certain price point).
Its also worth considering that an upgraded frame can also offer additional benefits such as improved comfort, and that if you do your homework re. frame geometry; reach and stack etc it becomes a lot easier to compare one frame to the next and take the guess-work out of looking at alternatives.
End of the day if you're going to spend the money on new gear, most cycling kit seems to be a money pit anyway in terms of depreciation / used value
EDIT: To use my own road bike as an example, it's had some decent bottle cages fitted and 105 calipers as the original Tektros were crap, but other than that the economically-sound choice is to leave the original Tiagra in place. I could spend around £800 on a full R7000 groupset, an appropriate BB conversion (it's currently PF30) and a better set of wheels with hubs that support the 11sp cassette, but all that would achieve in the real world would be one extra sprocket, a slightly lighter bike and maybe a few hundred quid added to its value (although if selling I guess it could be sold off peacemeal for a bit more, if you could be arsed). Better ways to spend the money IMO!
Saddles, bars, Fill ya boots.
