it is a 'less than 2%' difference, increase in size, which is quite acceptable when changing sizes of wheel/tyre. you speedo should be more accurate now! as mentioned, i suspect it is more an illusion that the car is 'raised', cos the difference in overall height is under 1cm.
but 'yes', you can add lowered springs to the original dampers. BUT, the ride quality may not be so good. however, it does depend on many factors, so it may not be too bad, and could actually be better! such things as spring rate (harness) and whether the new vs old are 'linear' or 'progressive'. the Eibach Pro Kit springs are highly recommended by those who have them; their springs are typically progressive rate, meaning the initial compression is relatively soft, then they become harder the more you compress them (ie: the softer coils complete their compression, leaving the harder coils to take over...in simplistic terms). you'll notice on progressive springs, the coils are not evenly spaced apart, where they are in linear springs.
anyway, i had Eibach Pro Kit springs on my 156 with stock dampers for its first 100K km's. it was quite acceptable. but after upgrading to KONI Sport, there was a major improvement.
so, 'one day', it's definitely worth upgrading the dampers too, but if $$ is a factor, you can start with just the springs.