Simple test, set the frame limit within FSX back to a limited value and see if the stuff gets better then. The external limiter only limits towards the upper range of the value, so choose something well lower than his setting and be limited again.
Bit of a downside of that unlimited setting is the increased possibility of blurry textures. So the sharper image would appear when going back to limited as more time is spend on e. g. textures.
If you are happy with the fps but get blurry stuff, maybe sacrifice some of them to see sharper textures.
Another thing would be to keep the disc away from high fragmentation and to disable all sceneries you don't intend to fly on that special flight. This will not only help with blurry textures but also with the overall loading times.