That is really disappointing that you have to be within the park to make FP+ reservations.
We were planning to take an afternoon break every day. Most of the time we won't be to the second park until 5 pm or later. By then are there really any good FP+ choices left? I can't imagine there would be.
First week of June, we had really good luck with extra FPs, particularly at MK. On our first full day, we used 11 FPs total, so we were able to book 8 additional FPs at the kiosks. The only headliner we booked at the kiosks was BTMRR, but there were others available. Many of our FPs were booked well after 5:00.
At EP, we got TT for early evening, but we did book that around 11:30. I think by 5:00, the "good stuff" would be long gone. Same with HS. We booked RNRC for 6:00 at 1:00. By 5:00, you'd have to get lucky and snag a cancellation. We didn't use extra FPs at AK, so I can't help with that.
Here is my "raw data" on extra FPs
June 1st--MK
LM at 2:20 (booked at 1:38; SB was 25 mins)
Barnstormer at 4:05 (booked at 3:35, SB was 30 mins)
WtP at 4:45 (booked at 4:15, SB was 40 mins)
Mickey at 4:55 (booked at 4:35; SB was 25 mins)
JC at 6:50 (booked at 6:38; SB was 25 mins)
PotC at 7:25(booked at 7:17; SB was 15 mins)
BTMRR at 9:30 (booked at 7:45; SB was 30 mins)
Tinkerbell at 10:30 (booked at 10:26; SB was 10 mins)
June 2nd--EP
TT at 5:00 pm (booked at 11:45; could have had 1:55, but had 2:00 ADR)
Character Spot at 12:30 (booked at 12:15; used RS for Soarin so one of us showed that we hadnt redeemed a FP; SB was 15 mins)
Maelstrom at 7:25 (booked at 7:15; SB was 20 mins)
June 3rd--EP (hopped from AK)
Maelstrom at 7:35 (booked at 5:15; could have had earlier time but had ADR at 6:00; SB was 20 mins)
June 4th--MK
PotC at 3:45 (booked at 3:30; SB was 20 mins)
Left park for Sanaa ADR
June 5th--HS
GMR at 2:05 (booked at 1:00; SB 10 mins; half of group)
RNRC at 6:00 (booked at 1:00 pm; SB was 45 mins)
June 6th--MK (hopped from AK)
Buzz at 7:35 (booked at 7:29; SB was 25 mins)
PotC at 8:10 (booked at 7:58; SB was 15 mins)
As you can see, some of the return times were really quick. My favorite was Buzz at 7:35. We booked that at the kiosk at 7:29. With the grace period, we literally booked the FP, parked our stroller, and hopped into the FP line. The SB wait was 25 mins, so it was definitely a good time-saving FP. Some of them had short SB lines, so we didn't save as much time. However, since FP takes priority over the SB line, we still grabbed what FPs were available, unless the kiosk lines were long. On most days, we could have grabbed even more FPs than we did, but we had ADR or other plans or a sleeping toddler.