i've got cheap coilys on my car, spent £220 delivered. to be brutally honest they are firm, not so firm my cd player skips on bumps, but when shooting down motorways, and dips make the ride pretty bouncy.
personally, i love the ride - even though it is quite firm, but my sisters new beetle has the same characteristics with a much more comfortable ride.
lowering a car usually induces a choppy ride anyway - i've never been in a car with a koni/fk/billy setup so i have no real comparison.
i'm based in glasgow if you want a ride-along and you can see for yourself