We wondered about this approach as well.
We most often planned our 3FP approx.: 10am, 11am, and noon.
One small technique we devised was to hold off on using the first FP until the END of our time window, to max out the early a.m advantage. That also enabled us to use the 2nd FP just AFTER our first.
On departure day in 2014, we tried early FP. I don't think it was an advantage to book them so early, because all the good FP were gone by the time we finished using our 3. Using our 9:20-10:20 FP at 10:15am = Space Mtn both standby and our first FP. At 9am, the FP wasn't much of a plus, but at 10:15am it was. I think we managed 3 Space Mtn total including FP. Then buzz, and our 2nd FP BTMRR, our 3rd was Splash. (That was not so good plan because there was a big gap to fill. I'm not sure how we checked...I think we used the gap between 2nd and 3rd FP usage to look up alternatives, or to make Splash earlier. I recall though that all the mtns were gone by then, so we kept Splash.
To your second question...it depends on kiosk lines- I think. As FP+ progresses kiosk lines have been greatly reduced. When FP+ was new- the kiosk lines themselves were over an hour long. On our last visit in January, some kiosks still had healthy waits in the 10am hour. By noon no line, but down to tier 2 options in MK, like JC. Going for 4th FP in other parks not worthwhile, IMO.
One more experiment. That trip, we changed plans. One day we'd planned mostly non park day with later PM FP in MK, including 7D. We added HS, but didn't want to give up our primo MK FP. The night before I looked up HS FP options and not much available.
Just for info, we checked again in the morning via kiosk. By then, pickings in HS were very slim! Best options was late GMR. No RNRC, no TSMM, no Frozen sing a-long. Even GMR almost gone! About noon, the Animation class had 2+ hour wait!!!! YIKES! That's the worst I've ever seen! At that point, we gave up.