Would it have been feasible to install 'conformal fuel tanks' on the Mercure?
Adding conformal tanks will add drag and weight. Each on its own will impact performance but both together will definitely have a huge impact and I doubt that it would even get airborne.

The reason that jet fighters have these is for ferry flights eg deployment overseas.. and since they usually leave the bombs/missiles/etc behind on these ferry trips there is not a big increase impact in take-off weight.