You don't need to wait until the whole toolpath is generated again, just ignore the toolpath generation and make any changes as if it was not there. The toolpath generator is a separate background process that is running on the lowest priority which should not hinder anything else you do in Cura. For that reason you have to make a very strong case before the lead developer of Cura would even consider implementing you feature request.
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Blizz 53
I've asked for http://umforum.ultimaker.com/index.php?/topic/6428-manual-refresh-of-render-time/ a while back.
The best solution that came out is to do all your settings with an empty buildplate, which I now do (if I remember)
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