It is impossible to beat the original rubber spring cones for all-round performance and ride. They are a true 'rising rate' spring with the correct rate-curve for the Mini.
For optimum performance, yes. But few drivers are looking for the ultimate performance in their daily drives.
This topic springs up (see what I did there) about once a month and there's always the same division of opinion often unobjective where people make statements about the unsuitability of spring conversions without any first hand experience of the product. Compared to many people who do use these conversions on their daily use car to great effect both with and without passengers and varying loads with no ill consequence.
If you're not looking for best performance handling from your mini but something that is a little more compliant and comfortable to drive on our pot hole laden highways and bi ways, coil conversions are a great option.
They do what they aim to very well, which is give a far improved ride and better road use characteristics equal or above fresh rubber and far beyond worn, with a much greater functional lifespan. But they won't offer the performance of a rubber donut.