Looks like nobody else has an opinion.  So, I'll jump in here.  It depends on which Summa plotter you have.  I have a D120.  I think it plots between 4" and 47.2".  Your D140 is larger.  If you can print from your version of Flexi, then why not print and plot from Flexi?  I do it all the time with my Edge 2 and Summa.  And, if you exported the file from Flexi to Gerber software, I don't think Gerber software can put Summa registration marks on the print.  Flexi can put the Summa registration marks on the print.  Hope I'm not reading your question wrongly.