Are coil-overs drastically better than custom upgraded shocks & springs?

 

Coilovers are adjustable for height/dampening. Normal shocks are not. If you're considering tracking or lowering your Mini, coilovers are the way to go. If you want to maintain your stock ride height/quality, normal shocks are the ticket.

 

What do you define as "custom upgraded shocks and springs"?

That, and what coilovers you're considering, will determine the answer.

 

That would be Koni's or Bilstein and HR springs or equivalent ...

 

I have the HR lowering springs and Koni combination, while the car handles good and the extra stiffness helps to "feel" what's gong on with the road, you will give up ride comfort compared to a good Coilover.

 

Given comparable price levels, the coilovers will either be sub-par or equivalent to your spring / strut combination choices.

Coilovers have the potential to go to much greater levels of mechanical advantage by tech / design.

If you currently have a Bilstein or Koni / H&R combo, then that's as good as it gets without going to AST/Moton, Ohlins, or that ilk. If you require / desire height adjustability, then there's coilovers available from Bilstein and H&R that are essentially the mentioned combos with threaded bodies.