There's many experts when it comes to suspension and many different brands, what suits one person may not suit another (or a certain car, due to worn bushes, previous accident damage, tyre and wheel size,tyre brands and design and many other variables), if you use a good lowering spring but have bad shocks the ride can become worse due to the spring overpowering the standard worn out shock, if you use a cheap lowering spring (sometimes) they use a softer spring than the standard one to decrease the ride height (rather than using a good design which costs more money to make), also the manufacturing process can make a difference to the ride quality , if a spring is made using inconsistent methods sometimes you can end up with uneven spring rates (particularly over time) which can also cause the car to roll and pitch through corners.
To answer the first question, measure the ride height of your car front and rear with a tape measure from the centre of the axle to the lip of the mudguard and work out where you'd like the vehicle to sit and minus the difference ie. -40mm front and -20mm rear, then check the ride height of the lowest point of the car 100mm at lowest structural point is as low as you can legally go (although this can sometimes be a grey area with some WOF testing), and then you can work out whether the lowering you wan't is realistic, also take into account that standard shock absorbers only have a certain amount of travel and if you go past this then you a gauranteed a harsh ride.