Now that Manual Cache is stable (so far, cross yokes and throttles), I’m back to testing caches offline for quality and performance.
Built several HQ caches; entire island of Manhattan, Las Vegas - most of the I-15 strip including McCarran up until the 515 interchange, Walt Disney World FL and Universal Studios FL (including the nearby hotels/villages) - each about 3-4 GB.
Ran a test flight by picking each POI as Departure using the same GA plane, time and WX identical. 1st run is with Data Settings of Internet, Bing Maps and Photogrammetry ON. Then I build the cache. 2nd run is with Internet OFF which disables Maps and Photogrammetry.
Local Cache appears to load and display LOD faster and better than with Internet, Bing and Photogrammetry ON. If you think about it, a lot of performance is server dependent, so one expects the cache to be somewhat better in this regard. Although I could not programatically recreate a flight plan that was identical in height, speed, etc., I did manage to pause several instances where the comparisons and field of view are close - screenshots of those show LOD holding up much better in the cache - i.e., buildings are fully formed and detailed, signs are sharper and more legible.
What the cache did not improve is draw distance and FPS. Even though all the possible data is local, your individual graphics settings still get a vote in what happens render-wise while in-flight. Things that were polygonal at a distance remained as such, offline or not - this was especially notable in Manhattan Island, with all the clustered buildings. As I get closer, LOD resolves - buildings become less polygonal, more formed and textured. As a rough estimate (I only have a rudimentary FPS counter - Steam), no real gains on FPS, maybe - just maybe, I see a boost of about 1-2 fps offline, which really tells me I need a better GPU. 
Maybe for the Holidays?
I’m not sure if the Program uses the Cache while Data Options are ON. I’ll need to experiment with that - maybe turn Internet ON but turn Photogrammetry OFF - make a note of current data usage counter and then see if it Increments significantly. If FS does use the cache, this could be a way for data-capped flyers to enjoy the photo-realism without incurring ISP expense.
I’ll update with more experimentation as long as the Manual Caching holds up.