I've seen an advert for HKS bump rebound adjustable coilover suspension.

What is 'coilover' suspension ?

 

All 'coilover' means is the coil-spring is sited over the damper to form an integral unit- as the Impreza's standard suspension is anyway. There really aren't that many cars currently manufactured that don't now utilise the concept (exceptions being some 4wd's[of the offroad variety],those using torsion-bar rear suspension,and a few others)
The term 'coilover' suspension is still often used in a generic motorsport context to refer to a spring/damper unit as described above but with an adjustable spring platform i.e. you can raise or lower the ride height and usually with a fairly standardised spring size (usually either 2 1/4" or 2 1/2" internal diameter) so that it is easy to change the spring and thus the 'rate'(i.e. effort required to compress the spring)

The HKS 'Hiper dampers and Super Dual Racing Springs' coilover suspension that you refer to is indeed quite an impressive bit of kit; As well as height adjustment there is bump and rebound adjustment(crudely-the rate at which the damper travels up and down)- most 'adjustable' dampers only permit rebound adjustment- and they include top-mounts(the bit that actually fits onto the car-body),that have spherical bearings(improve steering 'feel') and permit adjustment of camber and castor angles.
The only problem is that they come with a price-tag of £1700+vat.