Spoke to BBPT and they can do the Apex -60mm fitted for £360 (with CS discount).
Think it was David there, he recommends going for the -40mm's because going too lower could affect the driving in a negative way, ie excessive tyre wear, not the best handling, and a few other things I can't remember.
Their 172 turbo is on 15's and droped 40mm so I'm gonna go over and check that out in the morning.