...8J 18s on 225 40s + new std spec shocks with no problems.
I'm currently on Eibachs, approx 20mm drop, but have also run the same wheels with Apex & PI lowering springs both around 30mm drop. Only changed to Eibachs to try & get a bit more 'comfort' at slower speeds.
I had the same car on 9J 19s 235 35s & FK adj 'coil overs' but that combination was rock hard & the rears rubbed even without the plastic under guards(now fitted).
Its more the wheel/tyre width combined with the wheel offset(ET) & spacing that pose problems. Mine have an ET of 40, fr spaced 16mm, rears 12mm(16mm rubbed).
Lowering doesn't increase the wheel/suspension travel, in fact it reduces/eliminates the 1st part of travel. As long as you still have bump stops you should be ok but with the lower springs fitted the rear suspension sits on the bump stops before you even drive away. You can cut them down but I prefer to use the earlier TS stops as, although they're slightly shorter, they are stronger. Some people try without bump stops hoping the spring will limit the travel when it gets fully compressed but this won't/doesn't always work out soon enough.
It's a bit of trial & error but you definately CAN lower with 18s, expect a firm ride though & not a 2nd shock absorber
as someone on AO has posted.
I'm not getting into a long debate
about this but if you need help or advice please call me. I've done the theory/maths & it works.