OK, I have the IRIS F-15 downloaded and unzipped. It's a monster at 389Mb. How do I install it?

Exactly the same way as you installed the add-on's before.
Contents of effects folder copied into FSX\effects
Contents of gauges folder copied into FSX\Gauges (Though some install them in the aircraft "panel" folder)
open the folder "simobjects" should show "airplanes", open that, and you should have the F-15 aircraft folder, copy and paste that into FSX\Simobjects\Aircraft.
The thing about some of these releases that were FS9 add-ons is that they have a pile of Model (about 20) and textures (13). That's why the IRIS DL was so large.
The Merlin works except for one thing: From the *outside* the canopy is black, which I think looks kind of cool. Other than tht, I don't think there were any gauge problems, and works just fine in FSX.