50mm would be ok. Maybe lower on Coilovers.

The best option is the Eibach springs. You can buy them from Fords to no problems with warrenty. They only lower it 30mm, but are very highly recomended on FFOC. A big improvement espically with 17"s. 30mm is ok to still use standard shocks, but if any lower, the shocks would need to be upgraded.

My 2 Focus's both had the Eibach's. They were great. Looked fantastic even when I took the 17"s off and put the standard 15"s back on. Very low.

Andy