@abassign, if you really want to test aircraft, the maximum performance given in an FG context, then switch out to exterior view, disable all graphics setting(including ALS), enable wireframe mode(this one is most important, since it will let your GPU render both visible and hidden faces).
then, i want you to do this for me, just zoom in as close as possible not to obscure the osg stats, and do two screen shots: 1- fps only. 2- full osg stats.
i prefer that you turn draw-masks on, for Clouds and Terrain(since Terrain is different for each airport and we do not want sophisticated clouds).
i have a more powerful graphics card than you, and here are my results, it is important not to be biased, thus i have independently verified your testing method by replacing the model with UFO as well(using same method as renaming the files shown above).
all vertices rendered at once using "wireframe" mode in an exterior view zoomed as close as possible, i.e the ultimate cost of the model on the system(watch how frame rate drops "white line", and GPU time increase "orange" as i enable "wireframe"):

same as above, but fps only, shown just for clarification(even though the results are nearly equal):

now just to illustrate my point, here's the aircraft with no wireframe(see the fps increase because of the hidden faces? this is why it remains subjective without wireframe and dependent on view/camera, in addition, see the how the GPU "orange" time dropped)

exterior model replaced with UFO model, check property to see that it is indeed the Su-15 for clarity:

now fps only, and yes look how the osg stats are costly(390fps to 995fps just removing stats, and 1200fps without PUI), which is why Hooray is telling you to log the performance monitor to file instead of using GUI interface(for osgstats i have no alternative since it isn't exposed to the property tree, or else i would have recorded it to file):

finally, in comparison to previous screenshots, now comes the critical part that shows that fps in FG(or osg apps in general) depends on view and context, hence why i start with no Terrain/Clouds and enable "wireframe" mode, screenshot below shows the interior view, with the cockpit model removed(even with scenery/clouds, fps only: 192fps, osgstats: 64fps), the increase of fps is because of hidden faces:


since i did your method, can you verify that you see the same results with "my' method(using "wireframe", exterior camera just zoomed in, with osg stats, compared before/after replacing with UFO model)?