If you want wheels buy them. You don't need them. I recently bought a pair of Reynolds carbon wheels and new tires and I LOVE them. However, the .4 or .5mph I picked up is probably 65% due to the tires and 35% due to the wheels.
Tires (GP5k), aero helmet, proper saddle and seating position will all make a bigger difference than wheels. I do some long(ish) rides regularly > 40 miles sometimes 60+ miles and so knocking a couple minutes off my 60 mile time is meaningful when you're competitive like I am. However, I'm under no illusions that the biggest contributors to my speed increase of the last year are likely in this order:
Training
Tires
Positioning on bike (was already reasonably well positioned, this can make a huge difference if you're too upright)
Wheels