Of any potential upgrade, wheels make the most profound change to the performance of a bike. For example: take an old steel beater bike, and install some 1,200 gram carbon tubulars, and the bike is immediately transformed. Lower rotational weight is the key, and this means lowering the weight of the rims and tires.

Of course there are tradeoffs:

• Lower spoke count means more aero but require a higher rim weight.
• More aero rim profiles are faster at higher sustained speeds, but are more sluggish in start/stops/accelerations due (more weight).
• Lighter rims and tires are more fragile. Lighter tires wear out faster and suffer more flats.
• Carbon rims offer inferior braking, but are a lot lighter than alu (I own several of both).
• Tubular wheels offer an insurmountable performance advantages in terms of rotational weight. If you are riding for money, you will be riding on tubulars. If you don't want to deal with the expense and complexity of tubulars, then clinchers....
• For this reason, expensive clinchers are a waste of money - equivalent to blinging out a econo-box car.

In terms of 'ride quality' or 'road feel' or other such intanglible nonsense, tires make all of the difference here. Take any set of road wheels, and install a set of fat tires at a lower pressure, and a formerly jarring ride will become plush. Wheels have essentially zero vertical compliance. Tires, the saddle and seatpost (in that order) provide all of the shock absorption, vibration absorption etc etc..