On regular models the wing area is given by the total span times the average wing chord. And as mentioned it includes the part through the fuselage.

But on jets like the F16, F18 and F15 the side strakes and center body form such a large area that some allowance has to be included which makes the wing area calculation more difficult.

One thing for sure is that it makes the effective supporting area larger than it appears. And making new wings with minimally larger outer panels that still look like the originals is likely not going to do a whole lot. At most you'd likely be adding no more than 10% to the effective wing area. And that means a reduction in wing loading of only around 10% and an equivalent reduction in the landing speed.

If you're keen on making up new outer panels I'd suggest including mixing to allow the ailerons to deploy with the flaps as well as including leading edge slats to greatly increase the amount of wing camber. I think this would be your ace in the hole instead of some minimal amount of wing area gain. But it'll add to the total weight all over again. So it's hard to say if the gains would counter the losses.