Quick note just to save this small information. IBM support is telling us that it's impossible to use Commerce Developer with an external Solr server. After some research I found which config file to change in order to change which server it will call.
This file has multiple entries for configuring the search. Changing the com.ibm.commerce.foundation.server.services.search.url namebinding stringToBind to your solr server url will enable you to use an remote webserver in FEP7 and FEP8 atleast.