on a standard setup, the spring takes the main load directly into the "chassis rails", and only the damper load is going through the strut top.

with coilovers, you have the whole load going into the strut top area. i would want to make sure it's strong enough to take it over repeated loading.

what jim is talking about is you may need extra clearance for the increased diameter of the coil spring over the std damper on the inner arch area, and has nothing to do with the increased loads going through the strut top mount.